Waka Kotahi says two drilling rigs are in place to investigate a massive slip on State Highway 25A between Kopu and Hikuai on the Coromandel Peninsula.

The road was closed in late January after heavy rain caused it to collapse into a gully.

The Transport Agency says testing will help with an assessment of ground conditions within the slip area and the hillside above, to help determine a final repair plan.

It says field work should be completed this week, but surface conditions remain steep and slippery.

The site of the slip photographed in early February. Photo: RNZ / Libby Kirkby-McLeod.

Waka Kotahi says the data being collected will also help to determine the amount of excavation needed, as well as slope stability and foundation requirements for the different options being considered.

It says these are all important factors that help determine a solution that could be built quickly and efficiently.

Bore holes are around 50mm in diameter and will go to depths of around 20 metres.

The two rigs on site were drilling bore holes, retrieving cores of the soil from greater depths and providing more detailed soil and rock strength information.

The bore holes enable a core of the soil to be extracted.

These are being drilled at the sites of the potential bridge and bypass options.

The road is expected to stay shut for the rest of the year.

-RNZ.