The NZ Transport Agency is reassuring motorists that engineers are closely monitoring two cracks which have appeared on a section of State Highway 5 (SH5) in Tumunui, south of Rotorua, and the road remains safe for travel.

The damage is on a stretch of highway near the intersection of SH5 and Highlands Loop Road. The road runs through a historic crater known as Earthquake Flat.

The cracks stretch across both lanes of the highway and are about 2cm wide.

The Transport Agency's Bay of Plenty highways manager, Niclas Johansson, said geotechnical investigations were underway to determine the cause of the damage and to work out the best response.

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"These investigations will give us a better understanding of what we are dealing with and what the best solution will be going forward," he said.

"At this stage we believe that a natural process called tunnel erosion in the underlying soils caused the cracks in the road.

"Tunnel erosion is a known phenomenon associated with the volcanically derived soils common in the Bay of Plenty and across the central north island.

"The cracks appear to be a surface expression of the natural process that is happening beneath the road."

Mr Johansson said the safety of motorists was of the utmost importance to the Transport Agency and the site would be monitored constantly while a plan was developed.

"Alongside the monitoring we have closed one lane and stop/go traffic management is in place together with a 30km/h speed restriction. People travelling through the site are urged to adhere to the traffic management and to drive with extra care."

Mr Johansson said the Transport Agency would provide an update on the site in mid-July following the investigations.