State Highway 25A is reopening this morning following an investigation on Tuesday into cracks that appeared across the highway.

Thames-Coromandel District Council say Wednesday's reopening of SH25A between Kōpū and Hikuai from 7am until 7pm will mean traffic is down to one lane with a speed restriction of 30km per hour.

The affected area is close to the top of the summit from the Kōpū side.

"The highway will stay open only during daylight, until 7pm," says a council spokesperson.

On Monday January 16 Waka Kotahi NZ Transport Agency closed SH25A after the discovery of cracks in the highway following Cyclone Hale.

Large cracks appeared across SH25A. Photo: Thames-Coromandel District Council.

Geotechnical engineers carried out detailed investigations on Tuesday January 17 on a significant number of cracks, some measuring 15 metres deep and up to 75mm wide.

"Motorists will be directed to drive on the side of the pavement that is least affected," says a Thames-Coromandel Distrixct Council spokesperson.

"Cracks on this side were being filled on Tuesday afternoon to prevent rain getting in."

Council says the night time closure hour of 7pm will continue until further notice.

"Monitoring devices are being put in place to detect deterioration. If the situation worsens, the road may have to be closed again while further solutions are investigated."

While SH25A is closed, road users will need to use State Highway 2 through the Karangahake Gorge or State Highway 25 around the Coromandel Peninsula.

On Monday, SH25A was closed after cracks were discovered across the highway. Photo: Thames-Coromandel District Council.