Why the Upper Hutt Flood Mapping Consultation Must Be Officially Extended

The Upper Hutt Flood Mapping Consultation runs from 3 November to 12 December 2025, immediately after local body elections and during inaugural Council meetings leading up to the Christmas holiday period. At this early stage in the new triennium:

• Councillors are not yet fully briefed or available to engage with residents.
• Council committees and governance structures are still being established.
• Technical enquiries from the public are being redirected to GWRC or Wellington Water, not answered by Upper Hutt City Council.

This timing prevents meaningful public engagement and undermines democratic participation.

Key documents, including model build reports and modelling standards produced by GWRC, DHI, Tonkin & Taylor, and AWA Environmental, are not published as part of the consultation. Submitters must individually request them, meaning most residents remain unaware they exist.

Without these documents, the public cannot reasonably assess how the flood maps were produced.

Even straightforward questions, such as whether freeboard has been included in the suburban flood maps, are being processed as formal LGOIMA requests. In one example:

• Query lodged: 13 November 2025
• WWL’s advised response deadline: 12 January 2026

This is a full month after the consultation closes.

Such delays make informed submissions impossible under the current timeframe.

A review of the model build reports reveals:

• GWRC and WWL use different flood modelling standards.
• Hutt River calibration relies mainly on a single 1998 event and a few debris marks.
• Upper Hutt suburban stormwater models were not calibrated to any known Upper Hutt storms.
• Validation relies on scattered, one-line anecdotal reports (e.g., “driveway flooding,” “blocked sump”), not structured flood-event data.

There has been no systematic ground-truthing with the public after major storms in 1994, 1998, 2005, 2009, 2013, 2015, 2016 or 2019.

A long flood history with little public consultation

GWRC started studying the Hutt River at least 35 years ago in 1990. Since then, the Hutt Valley has experienced at least 9 or 10 major storm events:

There have been many opportunities for GWRC and WWL to model actual storm events in the Hutt Valley over the last 35 years and to consult the public after each large event to ground-truth modelled flood extents against what the community experienced. If the public can see that the flood models can simulate these observed events then the public can have more confidence that the models can reliably predict much larger events such as 100-year floods.

The current consultation repeats the known failings of the 2009 Pinehaven “drop-in” session:

• The public is asked to comment on 100-year, 330-year and 2,000-year floods, events they have probably not experienced.
• Drop-ins do not provide structured Q&A, formal records, or explanations of calibration and validation.
• There is no opportunity for the community to see whether the models match actual past flood events.

This falls short of the intent of NZS 9401:2008, which emphasises genuine community involvement at all stages of the flood modelling and mapping process.

A credible and transparent process requires:

• Extension of the consultation period
• Full publication of model build reports, calibration data, validation evidence, and modelling standards
• Formal public meetings with recorded questions and responses
• Councillor engagement once Councillors are fully available and briefed

Conclusion

The consultation, as currently timed and structured, is not fit for purpose.
An extension is essential to allow:

• informed public submissions,
• accountable governance, and
• confidence that the flood maps are based on reliable, properly calibrated and validated models.

We therefore urge the community to make submissions here requesting a formal extension of the consultation period.