Achieving good long-term environmental outcomes has been a big part of Te Aranui o Te Rangihaeata - the Transmission Gully motorway project.

Much of the landscape along the project route was highly modified by farming and forestry, resulting in the potential for increased erosion and run-off into local waterways, which flow into the Pāuatahanui Inlet and the Onepoto Arm of the Te Awarua-o-Porirua Harbour.

We continue to work with a range of specialists to understand and manage the potential impacts of work on these surroundings. As a result, we have the most comprehensive package of ecological mitigations and sediment management controls ever seen in New Zealand on a project of this type. This means that the effects of construction on waterways were minimised and then offset by a number of environmental improvements, with the harbour water quality improving over time. In less than 10 years, and once all construction is complete, the water quality and surrounding environment will be in a better state than before the motorway was built.

Applications to the Environmental Protection Authority (EPA)

Protecting our waterways

We know how precious the Te Awarua-o-Porirua Harbour, the Pāuatahanui Inlet, and surrounding streams are – techniques have been and continue to be used to ensure the effects on them during construction were minimised.

Our environmental management included:

  • 17 kilometres of silt fences
  • 37 kilometres of construction run-off collection channels
  • Over 100 sediment retention ponds that settled out sediment from construction run-off.

Ecologists, erosion and sediment control experts are vital to the project, ensuring procedures were in place for controlling and monitoring sediment run-off into watercourses and streams and identifying how earthworks were staged to manage the effects of the project, particularly on the Pāuatahanui Inlet and the Onepoto Arm of Te Awarua-o-Porirua Harbour. The Greater Wellington Regional Council also inspected the project’s sediment management every week.

Te Awarua-o-Porirua Harbour and Catchment Programme(external link)

Sediment retention ponds.

Futureproofing our natural environment


  • relocated 4,500 fish and eels from the Te Puka Stream into the upper reaches of the Wainui Stream in one of the most significant fish relocation operations in New Zealand. Ecologists removed the stream inhabitants from a 2.3-kilometre stretch of the stream that’s being diverted. Once the new Te Puka Stream channel has been constructed, fish, including koaro, redfin bully, and banded kokopu, will repopulate the new habitat.
  • relocated more than 50 lizards after dismantling boulder fields by hand on the slopes of the Te Puka and Horokiri streams. 46 copper skinks, six common geckos, and four brown skinks were housed in specially constructed ‘apartment cages’ at Ngā Manu Nature Reserve near Waikanae while a new boulder field was created. The lizards were then successfully relocated to their new homes.
  • ‘eco-sourced’ seeds for the project from Wellington, the Kāpiti Coast, the Akatarawa Ranges and Transmission Gully.

Once completed we will have…

  • enriched more than 27 kilometres of streams with riparian planting to provide shade and create better habitat for native fish, eels, birds and insects.
  • planted more than two million native trees and plants on the valleys and slopes surrounding the motorway, including seedlings of long-living tree giants like tōtara, matai and rimu. This will create one of the most significant lowland native bush areas in the Wellington region and reduce erosion and sediment entering streams and – ultimately – Te Awarua-o-Porirua Harbour.
  • improve fish habitats by building structures that allow native fish to travel upstream and under the motorway to breed. We do this by providing fish passage and replacing existing ‘perched’ culverts with waterways that allow fish and eels to move naturally up traditional migratory routes.

We expect...

  • with the increased indigenous habitat, iconic songbirds like tūi and bellbirds will successfully nest in the new bush. Hopefully, rarer species like native falcon (kārearea) and kākā will migrate from the Tararua, Rimutaka and Akatarawa Ranges to and from Kāpiti Island.

GreenroadsTM certification

The motorway has achieved a 2022 Greenroads silver pilot sustainability rating, a significant achievement for a roading project of this complexity and magnitude. This achievement was recognised at the 2022 Earth Day Annual Meeting of Greenroads TM .

GreenroadsTM website(external link)