Work is expected to start on the Awakino Tunnel bypass on State Highway Three between King Country and Taranaki later this year.
The 2.3km bypass will take traffic away from the single-lane Awakino Tunnel, built in the 1920s.
The new route will include two bridges over the Awakino River, and include a passing lane, landscaped rest area and walkway to the tunnel.
NZ Transport Agency systems manager Ross I'Anson said it was an exciting time to continue to make safety, resilience and route availability improvements on State Highway 3 from Awakino Gorge to Mt Messenger.
"The walking track, which has been included in the final design, is a fantastic feature as it allows people access to the old tunnel and to explore the historical area."
He said safety improvements would benefit the 2300 vehicles that travel on the road, of which 20 per cent are heavy vehicle.
"Although construction is not scheduled to later in the year people can expect to see work begin on site as geotechnical investigations start onsite," said Mr I'Anson.
Construction is planned to commence in October 2019, following the Transport Agency awarding the $28.9 million contract to Fulton Hogan.
The project is scheduled for completion in May 2021.