Letter to Upper Hutt City Council Leadership

UHCC Mayor, CE, GM Operations

Subject: Request to Include Pinehaven Stream Catchment in the Upper Hutt Flood Hazard
Modelling Consultation and to Extend the Consultation Period

To:

  • Mayor Peri Zee
  • Chief Executive Geoff Swainson
  • Group Manager Operations Tim Harty
    Upper Hutt City Council

From:
Flooding Us – A Community-Led, Evidence-Based Initiative

Date: 24 November 202 5

Executive Summary

We respectfully request that Upper Hutt City Council (UHCC):

  1. Formally request the inclusion of the Pinehaven and Silverstream flood maps in the current Upper Hutt Flood Hazard Modelling Consultation, and
  2. Extend the consultation beyond 12 December 2025, to allow Councillors and the public adequate time and information to participate meaningfully.

The Pinehaven Stream catchment flood maps are 15 years old, contain documented technical
flaws, and underpin key planning and resilience decisions within UHCC’s jurisdiction. Their
exclusion from this consultation undermines the completeness and credibility of the process
and exposes the community to potentially avoidable flood risk.

Because this issue requires coordinated regional leadership, we are writing separate letters to
GWRC leadership and the Wellington Water Board and Executive, raising the same concerns.
In the interests of transparency, we will also publish this letter on our website along with the
parallel letters to GWRC and WWL.

Key Concerns

1. Pinehaven flood maps are outdated and technically flawed

Despite being 15 years old, these maps continue to influence planning decisions. The
underlying flood model was:

  • calibrated to only one event (23 July 2009),
  • based on no rainfall data from within the catchment,
  • based on modelled peak water levels lower than the recorded gauge level,
  • unsupported by any published maps or observed flood extents.

Independent hydrological analysis indicates that:

  • the 2009 flood was larger than the 25-year flood of 8 December 2019,
  • the modelled 25-year flow (21.0 m³/s) used for streamworks design is nearly double the
    observed 25-year flow (11.4–11.7 m³/s),
  • the model embedded incorrect rainfall-loss assumptions that overstated baseline
    runoff and concealed the effects of potential future development on the Pinehaven hills.
    2. UHCC planning decisions depend on reliable flood information

The Pinehaven model and maps materially affect decisions on:

  • growth planning (including GTC’s land),
  • infrastructure renewals and capital budgets,
  • Plan Change 50 and future plan changes,
  • climate resilience and community safety obligations.

It is not appropriate for UHCC to continue relying on maps built on outdated standards and
unverified inputs.

3. The current consultation presents itself as covering “Upper Hutt flood maps” — yet
Pinehaven and Silverstream are excluded

This exclusion is not explained in the consultation material.
At the final drop-in session, staff indicated only that Pinehaven was “not in the programme for
renewal”, which does not justify excluding an entire catchment with known inaccuracies.

4. Councillors have not yet been briefed

The consultation opened three weeks after the 2025 local elections, during a period when
Councillors are attending inaugural meetings and have not yet received technical briefings. This
timing limits their ability to engage with the community or the consultation.

5. UHCC has statutory responsibilities for public safety and risk-informed planning

Under the Local Government Act, CDEM Act, and NPS-UD, UHCC must ensure that planning
and infrastructure decisions are supported by reliable hazard information. Continuing to rely on
known-flawed flood maps exposes the community and Council to avoidable risk.

Requested Actions

We respectfully request that UHCC:

  1. Formally instruct GWRC and Wellington Water to include Pinehaven and
    Silverstream in the current consultation to ensure the catchment is reviewed
    concurrently with the rest of Upper Hutt.
  2. Extend the consultation period beyond 12 December 2025, allowing Councillors,
    residents, and technical experts time to understand the modelling and make informed
    submissions.
  3. Provide public clarity on which agency is responsible for decisions regarding
    inclusion or exclusion of catchments in this consultation, to improve transparency
    and accountability.

Conclusion

UHCC’s leadership is crucial to ensuring that this consultation is complete, transparent, and
technically sound.

Excluding Pinehaven and Silverstream, despite long-recognised issues in the model and maps,
places future planning, infrastructure investment, and community safety at risk.

We urge UHCC to ensure Pinehaven Stream catchment is included in the consultation and that
the consultation period is extended accordingly.

We would welcome the opportunity to discuss this with you at your earliest convenience.

Kind regards,

Flooding Us
A community-led, evidence-based initiative

Kind regards

Stephen Pattinson
Director
Flooding Us NZ Limited