Thieves and vandals have shut down the use of high-tech road safety signs on one of New Zealand's most deadly highways.

The weather-activated speed signs line a 12km stretch of State Highway 29 in the Kaimai Range.

The signs were installed in November 2015 and were part of a two-year trial which had just gone into a data analysis phase. However, over the past seven days, 26 batteries in the signs have been stolen. The batteries weigh 75kg while battery storage cabinets, cameras and other power sources for the signs were destroyed or damaged.

The damage is estimated to cost tens of thousands of dollars.

New Zealand Transport Agency senior project manager Angela Crean said the agency was very disappointed, "especially because they were put in place to reduce the number of people being killed or seriously injured on the Kaimai Range".

In the past five years, there have been nine deaths on that stretch of SH29.

"The signs encourage people to drive at safe speeds that are appropriate to the road conditions when rain, ice and fog hit the Kaimai Range."

Seventy per cent of all crashes on the Kaimai stretch of SH29 were in wet weather, of those 40 per cent were caused by drivers travelling too fast for the conditions.

Crean said the equipment was specialised and took time to source.

Results from the first year of the trial showed there was a reduction in the number and severity of crashes. Given the positive impact the signs had in the first year it was decided the signs would stay in place until the outcome of the trial was known, Crean said.

Western Bay of Plenty acting head of road policing Sergeant Wayne Hunter confirmed police were investigating and there have been no arrests yet.

It is the second time the signs have been targeted, after vandalism in March.

Kaimai weather-activated variable speed limit

This signs trial is the first project of its type in New Zealand. The aim is to achieve safe and appropriate speeds in adverse conditions. If the trial is successful, and there is a reduction in death and serious injuries within the trial site, it may be rolled out across other similar sites in New Zealand.