Part 4: Better understanding of stormwater systems is needed

Managing stormwater systems to reduce the risk of flooding

In this Part, we discuss the three councils':

Better asset information is needed

Having robust information about physical assets and their condition helps public organisations make effective and sustainable decisions about how to manage those assets. Public organisations might not have detailed information about all of their assets, but they should have a good understanding of the most important assets.

If a public organisation does not have a good understanding of its most important assets, particularly the condition of those assets, it risks making poor long-term decisions. Long-term planning that is based on inaccurate information or poorly informed assumptions could result in costly or unsustainable decisions.

To assess the three councils' understanding of their stormwater assets, we analysed the data in their asset register against the primary asset attribute data that the International Infrastructure Management Manual considers should be collected. The primary asset attribute data are:

  • type;
  • material;
  • dimensions;
  • quantity; and
  • construction date.

We found that the three councils generally collected this data for their stormwater assets. However, in some instances, the data in the asset register was incomplete. For example:

  • Dunedin City Council did not have a complete data set in its asset register about the material of its stormwater mains.
  • Porirua City Council did not have a complete data set for the construction date of some of its stormwater assets.

The three councils also have limited knowledge about the condition and performance of their stormwater assets. For example:

  • Thames-Coromandel District Council does not know the condition of 64% of its above-ground stormwater assets and 75% of its stormwater pipes.
  • Dunedin City Council does not know the condition of about 90% of its piped stormwater network.
  • The three councils described their knowledge about the performance of their stormwater assets as unknown, less reliable, or uncertain.

The three councils have acknowledged the need to improve their asset data. Improved asset data on stormwater assets would allow the three councils to better identify and prioritise their work programme to achieve the levels of service and the cost of doing so.

Our findings are consistent with the results from Local Government New Zealand's 2015 National Information Survey, which revealed that a large proportion of water supply, wastewater, and stormwater assets are ungraded and that some councils' entire networks have not been graded according to their condition.

In our recent work, we have reported on the importance of accurate and reliable asset information and the need for councils to better understand the condition of their assets.10

Incomplete understanding of the capacity of the stormwater system

To date, the three councils have had an incomplete understanding of the capacity of their stormwater systems to handle current and future demand.

Councils need to have a good understanding of the capacity of their stormwater systems to handle current and likely future demand so they can ensure that levels of service will be met, now and in the future.

Thames-Coromandel District Council and Porirua City Council have completed limited assessments as part of specific investigations. For example, Thames-Coromandel District Council assessed the stormwater system for Whitianga by modelling its performance for different land use and rainfall scenarios.

As part of its integrated catchment management plans (see paragraph 2.8), Dunedin City Council has assessed the capacity of its stormwater system to handle current and future demand, including the effects of climate change and land use changes. However, it has not assessed all of its stormwater systems.

As described in paragraphs 2.14-2.15, Thames-Coromandel District Council and Porirua City Council are working towards improving their information about their flood risks. Doing this work will also improve their understanding of the capacity of their stormwater systems to cope with flood events.

A better understanding of the constraints on the three councils' stormwater systems' capacity would help identify and prioritise the work needed to achieve their agreed levels of service, now and in the future.

Lack of meaningful performance measures

The three councils' reported performance measures do not provide a meaningful indication about each stormwater system's performance against what it is designed to achieve. This limits the ability of people to understand the performance of the stormwater system.

Appropriate monitoring of the performance of assets is needed to ensure that services are provided as intended. This is supported by measurable and relevant performance measures.

Since 2015/16, under an amendment to the Local Government Act 2002, the Secretary of Local Government (who is the chief executive of the Department of Internal Affairs) has required all councils to report on five performance measures for managing stormwater, covering four aspects of their performance (see Figure 3). The purpose of the indicators is to provide standard, non-financial performance measures so the public can compare service levels.

Figure 3
The mandatory performance measures for managing stormwater

Aspects of performance Performance measure
System adequacy The number of flooding events that occur in a territorial authority district.

For each flooding event, the number of habitable floors affected (expressed per 1000 properties connected to the territorial authority's stormwater system).
Discharge compliance Compliance with the territorial authority's resource consents for discharge from its stormwater system, measured by the number of:
  1. abatement notices;
  2. infringement notices;
  3. enforcement orders; and
  4. convictions;
received by the territorial authority in relation to those resource consents.
Response times The median response time to attend a flooding event, measured from the time that the territorial authority receives notification to the time that service personnel reach the site.
Customer satisfaction The number of complaints received by a territorial authority about the performance of its stormwater system, expressed per 1000 properties connected to the territorial authority's stormwater system.

Source: Department of Internal Affairs.

Dunedin City Council and Porirua City Council report on their own measures as well as the mandatory measures. These includes measures on resident satisfaction with the stormwater system, the number of blockages, and the effect of stormwater on the quality of water in the receiving environment.

We did not see any performance measures from the three councils that provided a meaningful indication about each stormwater system's performance against what it is designed to achieve, including the mandatory performance measures.

For example, in its 2018-28 long-term plan, Thames-Coromandel District Council has the performance measure targets of zero flooding events and less than one habitable floor being flooded for stormwater. If the district experiences a flooding event that is larger than what the stormwater system is designed to handle, the Council will not achieve its performance measures.

This would happen even if the stormwater system performed exactly as designed. Therefore, the performance measures do not inform people about whether the stormwater system is achieving what it is designed to do.

Other issues with the mandatory measures include different interpretations of what a habitable floor is, and the variability about which flooding events councils consider when measuring the number of flooding events.

The three councils, and other councils we know of from our other work, have had difficulties in collecting accurate and complete information for these measures.

Thames-Coromandel District Council disclosed in its 2016/17 annual report that, although the district had suffered several large weather events, only one instance of habitable floor flooding was reported.

The Council stated that many calls went straight to emergency services or civil defence, rather than through its "Request for Service" system. This means that the Council was unable to accurately determine how many habitable floors were flooded.

A staff member from Dunedin City Council told us that people did not want to tell the Council about flooding because they were afraid it would be recorded on their land information memorandum, potentially affecting the value of their house.

The issues we identified with the mandatory measures are not limited to those for stormwater. We have previously reported that councils do not report on all the water supply indicators in the same way and that there are gaps in their information.11

In our view, the performance measures do not provide a meaningful indication about each stormwater system's performance against what it is designed to achieve. This limits the ability of people to understand the performance of the stormwater system.

Councils need enough information to ensure well-informed decisions

The three councils need to address the issues we identify in Parts 2-4 to ensure that they have enough information to make well-informed and deliberate decisions about their stormwater systems.

We have previously reported on weaknesses in councils' understanding of their assets. Our 2014 report Water and roads: Funding and management challenges noted that many councils did not have a complete understanding of the performance and condition of their assets. Some councils had low confidence in the reliability of their asset data.

Further, when we looked at all councils' infrastructure strategies for their 2015-25 long-term plans, more than half of them discussed the need to collect better information about assets. Underground networks posed the greatest challenge in terms of asset condition information.

Our 2018 report Managing the supply of and demand for drinking water highlighted that gaps in data can reduce the reliability of planning documents, make it harder for council management to provide advice to councillors, and make it harder for a council to make informed decisions.

Councils should prioritise gathering more information that helps them better understand their flood risk and the performance of their current stormwater systems in reducing that risk. This would then enable councils to identify the assets most important in protecting their community from flooding and the assets that they need more certain and relevant information about to ensure well-informed flood and stormwater-related decisions.

Until the three councils make these improvements, they will be making decisions based on inadequate information. In our view, the three councils should give priority to improving their information so that they can make well-informed and deliberate decisions.

Recommendation 4
We recommend that councils improve their understanding of their stormwater systems, which will entail ensuring the adequacy of their stormwater asset data, including condition data and information on the performance and capacity of the stormwater systems.
Questions to consider
For councils:
How do you know whether your stormwater system is delivering the level of service it was designed to deliver?
For people to ask their councillor:
Is the stormwater system delivering the level of protection it was designed to provide?

10: For example, Matters arising from the 2015-25 local authority long-term plans (November 2015), Getting the right information to effectively manage public assets: Lessons from local authorities (November 2017), Managing the assets that distribute electricity (June 2017), and Managing public assets (June 2013). These reports are available on our website,

11: Office of the Auditor-General (2018), Managing the supply of and demand for drinking water.