A replacement for State Highway 1 over Northland's notorious Brynderwyn Hills will be built just to the east of the current road - a major change from the original plan of building the new four-lane highway well to the west of the slip-prone hills.

Transport Minister Chris Bishop and Regional Development Minister Shane Jones announced Waka Kotahi's preferred route for a Northland Expressway on Wednesday, stretching about 100km from the motorway's current endpoint at Warkworth all the way to Whangārei.

The biggest surprise is that NZTA has dropped its earlier preference for a route that would have skirted around the western side of the troublesome Brynderwyn Hills.

Instead, the new route will, like the current highway, go over the top of the Brynderwyns, but to the east of the existing route.

Bishop said the Brynderwyn Hills section was highly challenging, due to the steep terrain and unstable geology.

"Alternative options looked at western routes but following further investigation, NZTA has reassessed and found a near-east alignment close to State Highway 1. This is a more direct route with more predictable geology that can be managed through engineering design," he said.

A slip on Brynderwyn Hills. Photo: Supplied / Waka Kotahi

A timeline and potential cost for the project are not yet known, but the full Northland Expressway is expected to be one of the most expensive infrastructure projects in New Zealand history.

Last year the Infrastructure Commission warned the project could consume $1 of every $10 spent by the Government on infrastructure during the next 25 years.

Plans for a new route around the Brynderwyns were put on ice in 2017 but revived with urgency in the wake of Cyclone Gabrielle in early 2023, when the highway was closed on-and-off for months due to slips and reconstruction.

During the closures, traffic between Auckland and Northland was forced to use narrow back roads via Waipū, or the much longer State Highway 14 around the west coast.

Even the $85 million repair job carried out on the unstable south side of the Brynderwyns in 2024 is expected to last only another seven to 10 years.

A slip triggered by Cyclone Tam during Easter weekend, which blocked one lane and reportedly damaged a car, was another reminder of the route's instability.

Also announced this morning were the preferred routes for two other sections of the Northland Expressway.

Bishop said the 26km section from Warkworth to Te Hana, bypassing the summertime traffic jams of Wellsford, would be built mostly to the east of the current highway.

Italian company Webuild confirmed earlier this year it would bid to build and operate that section of the Northern Expressway as a public-private partnership.

The section of highway from the northern side of the Brynderwyns to Port Marsden Highway at Ruakākā would run west of the existing highway.

A preferred route for the northernmost section, a notoriously crash-prone stretch of highway between Ruakākā and Whangārei, had yet to be determined.

The highway through urban Whangārei would retain its current route but would be widened in places.

This story was originally published by RNZ