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Waka Kotahi says it needs to fully close State Highway One over the Brynderwyn Hills to fix storm damage on the vital connection for Northland.
A spokesperson for the agency said closure dates and detour routes would be confirmed next month.
"We will work around key dates such as Waitangi Day and Easter and... will engage with our partners, stakeholders and the community to ensure the work is carefully planned to minimise disruptions as much as possible."
Long detours were in place after Cyclone Gabrielle caused several huge slips over the Brynderwyns, which were closed, off and on, for weeks.
Waka Kotahi's board is due to sign off recovery and rebuild work in the Brynderwyns this month - but it is a short- to medium-term fix, which requires closing the highway.
"This recommendation seeks funding to ensure this vital connection for Te Tai Tokerau is maintained for the short to medium term," the spokesperson said.
"This recommendation would result in a full closure of SH1 Brynderwyn Hills in order to complete this work as safely and as efficiently as possible."
The agency said it was working to ensure alternative routes would be fit for purpose.
"A business case for long-term resilience options is still being considered, alongside the proposed recovery and rebuild works," the spokesperson said.
"As this is unfunded, long-term resilience options will follow our standard business case processes."
A consortium of businesses in Te Tai Tokerau/Northland is concerned that the lack of a reliable transport link between Northland and Auckland is holding back the region's economic growth.
Northland Corporate Group includes Channel Infrastructure NZ, Culham Engineering, Marsden Maritime Holdings, McKay and Northpower.
Cyclone Gabrielle caused huge slips over the Brynderwyns, which were closed, off and on, for weeks. Photo: RNZ / MARIKA KHABAZI
Its co-chair Andrew McLeod, who is also chief executive of Northpower, told Checkpoint that there needed to be urgent investment into critical road infrastructure to connect the region with the rest of the North Island.
The partial closure of SH1 at the Brynderwyn Hills was costing Northland $2 million a day, and Waka Kotahi needed to come up and take the time to talk to businesses about long-term solutions, he said.
"Brynderwyn was never an ideal route, and it's just got worse and worse.
"We can't have that corridor compromised long-term. We've got to get past this patching idea and look to long-term solutions.
The connection was important not just for tourism, horticulture and agriculture but also the transport of construction materials to Auckland, he said. Any closure needed to be timed carefully and take these industries into account. "We can't afford to get that window wrong."
Northland has beautiful beaches, like Matai Bay on the Karikari Peninsula. Photo: 123RF
Northland was still open and accessible to tourists despite the disruption to SH1, he said. There were still corridors suitable for light traffic to the east and west of the highway - with "quite nice scenery".
"A lot of people decided they couldn't get to the north, or thought they'd get stuck here... But we're open. Come up - it would really help our businesses here.
"The winterless north feels very much like the isolated north, and we want to make the case to fix that. But first we need to demonstrate the economic prosperity that can be unleased with robust roading links and that's why we are calling on all businesses to complete our survey, which will support a case for investment."