Motorists will be shifted to the new road on the Brynderwyn Hills early next month as work to improve safety cranks into top gear.

The $18 million New Zealand Transport Agency project started in December 2014 and is on target for completion before the end of this year.

NZTA's Northland highway manager Brett Gliddon said motorists would notice changes on that stretch of state highway from the beginning of May.

After the summit and bottom sections of the new road alignment were finished, due by the beginning of May, he said traffic would be moved down from the existing road to the new road.

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"Southbound traffic will be moved first, followed by northbound traffic. This is expected to take about a week and there will be speed restrictions to keep motorists and those working in the area safe.

"The new road will have a temporary surface which will be completed once the existing road is lowered to reach the same level as the new alignment. This will allow the new road to be widened to its final width," he said.

Mr Gliddon said while the changeover would be disruptive for drivers, the final result would help make the stretch of road safer and more resilient.

"This will reduce the risk of crashes and minimise the impacts of vehicles crossing the centre line, as well as building greater resilience on the network by reducing the number of traffic delays which result from crashes - all part of our continued investment in Northland's highways."

He said earthmoving was continuing on the middle section of the hill and, weather permitting, this was expected to be completed in December.

The volume of earth being moved is equivalent to 146 Olympic-size swimming pools.

Mr Gliddon said 25,000m3 of soil was being removed and gullies were being filled with 340,000m3 of soil.

"The earth then needs to be levelled in layers. Each layer is then tested to ensure it forms the best foundation for the new road surface," he said.

"A wetter than expected January delayed earthworks while the ground dried out, however the project is still on target for completion before Christmas 2016."