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.