Letter to Wellington Water Limited Leadership

To Wellington Water Board, Wellington Water Committee, CE & COO

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

To:

  • Wellington Water Board Chair – Nick Leggett
  • Wellington Water Committee Chair – Cr Ros Connelly
  • Chief Executive – Pat Dougherty
  • Chief Operating Officer – Charles Barker
    Wellington Water Limited

From:

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

Date: 04 December 2025

Executive Summary

We respectfully request that Wellington Water Limited (WWL):

  1. Support the immediate inclusion of the Pinehaven and Silverstream flood maps in
    the current Upper Hutt Flood Hazard Modelling Consultation , and
  2. Support an extension of the consultation beyond 12 December 2025 , to ensure the
    process is robust, transparent, and informed by complete and accurate information.

Pinehaven Stream catchment is the only sub-catchment in Upper Hutt with a 15-year-old model
known to contain serious technical flaws, yet it has been excluded from a consultation
promoted as providing “Upper Hutt flood maps”. Given WWL’s responsibilities for stormwater
modelling, infrastructure planning, and catchment management, this exclusion undermines the
credibility of the overall consultation and has direct implications for network resilience and
downstream public safety.

We are simultaneously writing to UHCC and GWRC leadership about the same concerns, as
coordinated leadership across all three agencies is essential.
For transparency and consistency, we will be publishing these letters on our website.

Key Concerns

1. The AWA stormwater models are being finalised without consideration of the Pinehaven
catchment

The suburban stormwater models prepared for northern, central, and southern Upper Hutt
exclude Pinehaven and Silverstream. This produces an artificial “hole” in the district-wide
stormwater hazard picture.

Given Pinehaven’s well-known history of model inaccuracies, and its role as a major flow path
through Silverstream, excluding it creates an incomplete view of flood risk for both communities
and for the city as a whole.

2. The AWA models were not calibrated, only lightly validated using anecdotal
observations

The three Model Build Reports state that the models were validated by comparing model
outputs to small numbers of anecdotal resident reports such as:

  • “flooding from blocked drain”,
  • “flooding heading up driveway”,
  • “flooding from Gemstone Drive coming down Beryl Grove”.

These are valid supporting observations, but they do not constitute calibration, nor do they
represent systematic validation.

The consequence is that a large part of the city now has stormwater maps produced without
calibration, while the Pinehaven maps (with known flaws) are excluded entirely.

This fragmented approach does not provide the community or Councillors with the integrated
understanding of urban flood risk that WWL normally strives to deliver.

3. Excluding Pinehaven undermines stormwater network planning and hydraulic neutrality
assessments

WWL is responsible for ensuring stormwater networks can accommodate flows from both
current and future development.

The Pinehaven model currently in use:

  • significantly overestimates baseline flows,
  • was calibrated to an unverified and unrealistically low modelled water level for the 2009
    flood,
  • embeds rainfall-loss assumptions that treat forested hills as an already-urbanised
    surface,
  • overstates the 25-year flow at the gauge location by almost double when compared to
    independent assessments.

These errors impact:

  • stormwater network capacity planning,
  • assessments of development impacts,
  • the ability to set defensible stormwater neutrality benchmarks,
  • cost allocation (public versus developer), and
  • the design of downstream upgrades.

Given that WWL is responsible for advising on development impacts and three-waters planning
across Upper Hutt, the continued use of this flawed model exposes WWL to unnecessary risk.

4. Public consultation on “Upper Hutt flood maps” is incomplete and inconsistent without
Pinehaven

Residents were specifically invited to provide feedback on whether the maps “match lived
experience”.
Yet Pinehaven and Silverstream residents have no maps to review.

This undermines trust in the process and prevents meaningful public participation on one of the
most flood-affected catchments in Upper Hutt.

5. The consultation timing limits Councillor engagement

The consultation opened just weeks after the local body elections. New Councillors have not
yet been briefed, and are therefore unable to properly engage with constituents or the technical
content.

Given that stormwater modelling is complex and influential in future investment decisions, this
reinforces the need for an extension.

Requested Actions

We respectfully request that Wellington Water:

  1. Support the inclusion of the Pinehaven Stream catchment flood maps in the current
    consultation , so that Upper Hutt residents receive a complete and coherent
    understanding of flood risk across the city.
  2. Support extending the consultation period beyond 12 December 2025 to allow the
    public and elected members to engage meaningfully with the modelling information.
  3. Advocate clarity on decision-making responsibility by confirming publicly which
    agency (GWRC, WWL, UHCC, or jointly) decided to exclude Pinehaven from the
    consultation.
  4. Publish the model build reports in full on the consultation platform so the public has
    access to the technical basis of the maps.

Conclusion

WWL plays a central role in ensuring that stormwater modelling, infrastructure planning, and
public consultation processes are technically robust and reflect real-world catchment
conditions.

The exclusion of Pinehaven Stream catchment from this consultation risks undermining the
credibility of the wider flood mapping exercise, and it prevents residents from providing
feedback on the most disputed and technically flawed model in Upper Hutt.

We ask WWL to support the inclusion of Pinehaven and Silverstream in the consultation, and to
support an extension to ensure a complete, transparent, and technically sound process.

We welcome the opportunity to meet with WWL representatives to discuss this matter further.

Kind regards,

Flooding Us
A community-led, evidence-based initiative

Stephen Pattinson
Director
Flooding Us NZ Limited