Ngaio Gorge stabilisation

We are strengthening and stabilising the Ngaio Gorge slip to make this key transport route stronger and more resilient. The road will be down to one lane during construction, controlled by traffic signals.


July 2022

  • Lower slip anchor installation completed.
  • Mesh installation is 70% completed.
  • Attenuation fence foundation completed.
  • Upper slip anchor installation in progress.
  • Mesh installation is 50% completed.

March 2022

  • Three retaining walls including a steel guardrail and temporary widening of the road have been completed.
  • Slope stabilisation works on the main slip area are in progress and scheduled to be completed in June 2022. A limited number of highly trained specialised staff are engaged for these works due to health and safety risks prevailed in the slope area.
  • There is a 6 to 8 weeks delay in project progress due to Covid -19 lockdowns and Omicron outbreak. The new scheduled completion date is 22 December 2022.

December 2021

  • Three retaining walls were completed including anchors and roadside backfilling 
  • Installation of Steel handrail on Retaining wall no. 1 completed
  • Drainage work is in progress. 
  • Surface preparation and kerb and channel work are completed for the temporary widening of the road at new retaining walls.
  • Scaling work at the main slip and lower slip areas commenced and continued. 
  • TTM lane swap took place to facilitate the work at slope areas. 
  • The project will be shut down from Wednesday 22 December 2021 to Monday 10 January 2022 for the Christmas holiday period and two-way traffic is available during this period.

June 2021

  • Work continues to progress on construction of three retaining walls. 
  • Concreting work has been completed for two walls and the installation of anchors for one wall is underway.
  • Ground preparation for the third wall is finished and reinforcement work for this wall is in progress.
  • The road continues to operate with one lane and is being controlled by traffic signals.

April 2021

  • Work is progressing well on the three retaining walls.
  • Ground preparation for two of the walls has been completed and these walls are currently being reinforced.
  • The pile foundations (40 piles) are in place for the third wall and ground preparation is underway to build the concrete wall.
  • The road continues to operate with one lane and is being controlled by traffic signals.

March 2021

  • Work is proceeding well.
  • The pile foundations for two of the retaining walls have been completed, and the groundwork for construction of both retaining walls is underway.
  • Vegetation has also been cleared in preparation for building a third retaining wall, and drilling for the wall's pile foundations will start in mid-March.
  • Ngaio Gorge Road continues to operate with one lane and is being controlled by traffic signals.

December 2020

  • Construction is underway and most of the preliminary preparatory work has been completed.
  • Borehole drilling started – seven pile foundations for retaining wall number 2 are completed. This work is expected to be finalised by August 2021.
  • The site will be closed on from Tuesday 22 December 2020 for the Christmas holidays, and resumes on Monday 11 January 2021.
  • Ngaio Gorge Road is open for two-way traffic during the site closure period and will be down to one lane when construction work resumes.

Project programme milestones

  • Stage 1 – October 2020 to September 2021
    Construction of three reinforced concrete retaining walls on the loser side of the road. 
  • Stage 2 – October 2021 to May 2022
    Stabilisation of the two slip sites, including installation of rock anchors, netting, and MSE bund at the main slip area. Installation of an attenuation fence, anchoring, and drapery at lower slip area. 
  • Stage 3 –  June 2022 to October 2022
    Road upgrades, including new/upgraded stormwater infrastructure, street lighting, provision of parking spaces, and improvements to the footpath and uphill cycle lane.

About the project

As part of the Council’s programme of work to improve safety and accessibility of major transport routes we are committed to fixing the Ngaio Gorge slip.

Construction work is scheduled to get underway from 12 October 2020 and will give priority to the construction of three retaining walls to realign the road. Once that is completed the Council is to continue stabilising the lower and upper slip sites and remove the concrete barriers and shipping containers.

The project cost is $11m including professional fees.

Impact on traffic and residents

The road will be down to one lane for the duration of the construction and will be controlled by traffic signals.

There will be impacts on traffic while the work is underway. We will monitor traffic flows, particularly during the morning and evening commuter peaks to minimise the impact on people’s journey times.

Work hours

Crews will be working between 7am to 6pm, Monday to Saturday from 12 October 2020 until October 2022.

Alternative routes

We encourage people to use alternative routes such as the Ngauranga Gorge or Onslow Road to help ease congestion during busy times.

There are good options for public transport in this area. Please check Metlink timetable for options.


Cyclists are affected during the construction work of the project especially when they are travelling uphill. Safety must be the number one priority, so we kindly request cyclists to dismount and push their bikes through the pedestrian areas of the site when travelling uphill to minimise health & and safety risks.

Trelissick Park users

Trelissick Park will remain open during this time. The upper entrance at Trelissick Park will remain open during the project. Later on - the lower site entrance will open.

Parking for people going for a walk in the area will not be available closer to the site to ensure safe passage for pedestrians and vehicles passing through.


Pedestrian access will be provided throughout the project period.

Environmental considerations

An Environmental management plan has been developed to avoid contaminant into the stream below.

Also we have a Lizard Management Plan in place with the Department of Conservation to ensure safe relocation of all lizards in the area.

More information

If you have any questions about the Ngaio Gorge Slope Stabilisation Project please call 04 499 4444 or email us at