Crews using two portable geotechnical rigs have begun taking core samples along the preferred corridor of the new State Highway 3 replacement for the Manawatu Gorge.

Lonnie Dalzell, the NZ Transport Agency's Manawatu Gorge project manager, said in some cases the rigs would need to be transported by helicopter to positions that couldn't be safely accessed by heavy vehicles.

"This is an important part of the investigation process and an exciting step forward for the project.

"The samples we take will be used to get a better understanding of geology of the area and this information will help inform the detailed design process.

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"We are aiming to complete the sampling work over the next six weeks, but it is weather dependent."

In March, NZTA selected the preferred corridor from four options. It would run from near the Te Apiti carpark western entry of the closed Manawatu Gorge, cross the Ruahine Ranges north of the gorge, before emerging at Woodville.

"The Transport Agency project team is working on the planning and procurement of the next phases of the project with the construction of the new road scheduled to begin in 2020, and be completed by 2024," Dalzell said.