Waka Kotahi NZ Transport Agency 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 said 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 said field work should be completed next week, but surface conditions remained steep and slippery.

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

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

It said these were all important factors that helped determine a solution that could be built quickly and efficiently.

Boreholes were around 50mm in diameter and would go to depths of around 20 metres.

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

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The boreholes enabled a core of the soil to be extracted.

These were 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