Letter to Greater Wellington Regional Council Leadership

To GWRC Chair, Chief Executive & Environment Group Manager

Subject: Inclusion of Pinehaven Stream Catchment in the Upper Hutt Flood Hazard Modelling
Consultation and Request for Extension of Consultation Period

To:

  • Chair Daran Ponter
  • Chief Executive Nigel Corry
  • Group Manager, Environment Group, Lian Butcher
    Greater Wellington Regional Council

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

Date: 04 December 202 5

Executive Summary

We respectfully request that Greater Wellington Regional Council (GWRC):

  1. Include the Pinehaven Stream catchment (Pinehaven and Silverstream) in the
    current Upper Hutt Flood Hazard Modelling Consultation , and

  2. Extend the consultation beyond 12 December 2025 to provide residents, iwi,
    stakeholders, and Councillors with adequate information and time to make informed
    submissions.

The current exclusion of Pinehaven is inconsistent with the consultation’s stated objective of
“ensuring the maps accurately reflect local conditions before they are finalised” and conflicts
with GWRC’s responsibilities under the Flood Hazard Modelling Standard (2021), as well as with
principles of transparency, public engagement, and sound risk management.

We are writing parallel letters to UHCC leadership and Wellington Water leadership, as this
issue requires coordinated decision-making across all three agencies.

For transparency, we will publish all three letters on our website and provide them to our wider
community contacts.

Key Concerns

1. Pinehaven flood maps are outdated and technically unreliable

The Pinehaven flood maps (c. 2010) underpin planning and flood-risk decisions for Pinehaven,
Silverstream, and downstream areas. However, the underpinning model:

  • was calibrated to only a single event (23 July 2009),

  • used no rainfall data from within the catchment ,
  • replaced the recorded peak water level of 1.6 m with a modelled level of 1.2 m,
  • provided no published maps or verified flood-extent observations for the 2009 flood,
  • embedded incorrect rainfall-loss parameters treating forested hills as if they were
    already urbanised.

Independent hydrological analysis (R J Hall & Associates, peer-reviewed by Graeme Horrell) has
demonstrated that:

  • the 2009 flood was larger than the 25-year flood in 2019, and

  • the 25-year flow used for streamworks design (21.0 m³/s) is close to double the
    observed 25-year flow (11.4–11.7 m³/s).

These findings directly undermine the validity of the current Pinehaven flood maps and the
hydraulic neutrality assessments for potential future development in the upper catchment.

2. Excluding Pinehaven contradicts GWRC’s Flood Hazard Modelling Standard (2021)

The FHMS emphasises:

  • consistency across catchments,

  • transparent communication of model assumptions,

  • verification of model performance, and

  • appropriate use of hazard classification (including velocity–depth categories).

Yet in this consultation:

  • the Hutt River model (DHI/T+T) includes freeboard and uses hazard classifications,

  • AWA’s suburban stormwater models do not include freeboard and use a different
    methodology,

  • Pinehaven is excluded entirely, despite being part of the same district-wide hazard
    context.

This piecemeal approach is inconsistent with FHMS principles of integrated, consistent, and
transparent regional flood hazard modelling.

3. The consultation publicly presents itself as covering “Upper Hutt flood maps”

The consultation webpage states that residents can:

“look at the maps to see if they match your experience … Your feedback will help us ensure the
maps accurately reflect local conditions before they’re finalised.”

However:

  • Pinehaven and Silverstream are not included,

  • this exclusion is not stated or explained to the public,

  • at the drop-in session, staff offered only that Pinehaven was “not scheduled for
    renewal”.

This creates an information gap for the public and undermines the stated consultative purpose.

4. Residents were invited to “have their say” — but have no opportunity to comment on the
most controversial and technically disputed catchment in Upper Hutt

Given the known public concern about Pinehaven modelling, including its relevance for
downstream flood risk and future development, its exclusion effectively prevents meaningful
public participation in a matter of high community interest.

5. Councillors and the public do not currently have access to the model build reports

Model build reports were not published with the consultation.
They were only available upon request.
This limits transparency, especially in a technical consultation.

6. The timing of the consultation (immediately following local elections) restricts
Councillor engagement

Inaugural Council meetings are only now occurring. Many new Councillors are not yet briefed.
This further reinforces the need for an extension.

Requested Actions

We respectfully request that GWRC:

  1. Add the Pinehaven Stream catchment flood maps to the current consultation ,
    ensuring that all parts of Upper Hutt are treated consistently and that known flaws are
    addressed before decisions relying on them are finalised.

  2. Extend the consultation period beyond 12 December 2025 , allowing the public, iwi,
    Councillors, and stakeholders time to engage meaningfully.

  3. Clarify publicly which agency determined that Pinehaven would be excluded , and
    on what basis, to ensure accountability and transparency.

  4. Publish the full technical model build reports and supporting documentation for all
    models included in the consultation.

Conclusion

Pinehaven is the only Upper Hutt catchment with a 15-year-old model containing known errors
that significantly affect existing residents and influence future development assessments.
Its exclusion from a consultation presented as covering “Upper Hutt flood maps” is neither
technically defensible nor consistent with GWRC’s commitment to transparency and
community engagement.

We urge GWRC to demonstrate leadership and uphold the principles of the Flood Hazard
Modelling Standard by including Pinehaven in this consultation and extending the consultation
period.

We would welcome the opportunity to discuss these matters with GWRC representatives.

Kind regards,

Flooding Us
A community-led, evidence-based initiative

Kind regards

Stephen Pattinson
Director
Flooding Us NZ Limited