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Achieving good long-term environmental outcomes is a big part of Transmission Gully’s new motorway.

Much of the landscape along the project route has been highly modified by farming and forestry, and the resulting increased erosion and run-off into local waterways has been clogging up the Pāuatahanui Inlet and the Onepoto Arm of the Porirua Harbour ever since.

We work with a range of specialists to help us understand and manage the potential impacts of our works 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 will mean that the effects of construction on waterways will be minimised and then offset by a number of environmental improvements that will see the harbour water quality improve over time. In less than 10 years after the new motorway opens, 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 Porirua Harbour, including the Pāuatahanui Inlet and surrounding streams are – so we’re using techniques to ensure we minimise the effects on them during construction.

Our environmental management includes some:

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

Ecologists, and erosion and sediment control experts are vital to the project, ensuring procedures are in place for controlling and monitoring sediment run-off into watercourses and streams and identifying how earthworks will be staged to manage the effects of the project, particularly on the Pāuatahanui Inlet and the Onepoto Arm of the Porirua Harbour. The Greater Wellington Regional Council also inspects the project’s sediment management on a weekly basis.

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

Sediment retention ponds.

Futureproofing our natural environment

We have…

  • relocated 4,500 fish and eels from the Te Puka Stream into the upper reaches of the Wainui Stream in one of the biggest 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. The 46 copper skinks, six common geckos and four brown skinks, are being housed in specially constructed ‘apartment cages’ at Ngā Manu Nature Reserve near Waikanae. They will be returned to the area once a new boulder field is created.
  • ‘eco-sourced’ seeds for the project from Wellington, the Kāpiti Coast, the Akatarawa Ranges and Transmission Gully.

We will…

  • enrich more than 27 kilometres of streams with riparian planting to provide shade and create better habitat for native fish, eels, birds and insects.
  • plant around 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 – the 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 using bridges, minimising culvert lengths and gradients and replacing existing ‘perched’ culverts with waterways that once again allow fish and eels to move naturally up traditional migratory routes.

We expect…

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

GreenroadsTM certification

Transmission Gully will be the first motorway constructed in New Zealand to achieve GreenroadsTM silver certification – an international sustainability rating system for road design and construction.

GreenroadsTM website(external link)