This showed there was "clearly a need" for police to start issuing tickets, she said.
"It is not an advisory speed."
The tougher stance would be run in conjunction with the 4km/h speed tolerance that had been rolled out across the country throughout summer.
Ms Milham said drivers should treat the new signs like any other speed sign.
The speed signs were activated when their sensors detected rain, ice, wind or a change in visibility.
Ms Milham said operators in Auckland would look through a live camera feed to decide if the "weather had deteriorated to the point the speed needs to be lowered".
NZ Transport Agency project manager Angela Crean said feedback had been positive.
"Those that have contacted us have said that in treacherous weather they feel safer travelling at the lower speed over the Kaimai Range" Ms Crean said.
Ms Crean said the lowest speeds the signs would go down to was 30km/h but that would only be in incidents such as road works or crashes.
The weather-activated speed signs were installed in the Kaimai Range due to the high weather-related crash rate.
"Data shows that over 70 per cent of the crashes on the range happen in wet weather, and that over 40 per cent of these were caused by drivers travelling too fast for the conditions," Ms Crean said.
"The overall goal in the Government's road safety strategy, Safer Journeys, is to reduce the number of crashes. 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 similar sites around New Zealand."
Bay deaths
There has been 28 road deaths in the Bay of Plenty this year, compared to 16 in 2011.
