A road safety charity says a plan to increase the speed limit to 110kph on some open roads under a proposal to tailor driving speeds to road conditions is a bad idea.
The Transport Agency said some roads are now so well-engineered that it is time to change the law to allow for a 110kph limit.
It will also analyse which roads are at higher risk of accidents and serious injury and need lower limits.
In cases where limits are cut there is a promise that they will be increased again once upgrades have been carried out.
However road safety charity, Brake, said a speed limit increase would not promote safe driving.
Chief executive Mary Williams said there were no circumstances in which the 100kph limit should be raised to 110kph.
"The bottom line is that the faster we go the less time we have to react and it's not just determined on the type of road, and the road engineering it, it is also determined by the weather conditions and time of day, and types and levels of vehicles on our roads."
She also said 30kph was the absolute maximum that was safe for residential areas with pedestrians and cyclists using the road.
Limits on windier country roads will drop to 70kph and motorists around busy intersections will have to slow down to 30kph, under the plan.
Director of road safety at the Transport Agency, Ernst Zollner said the agency aims to make speed limits more consistent around the 88,000 roads it has classified, but most won't change.
"There are some roads in urban areas where we have protected cyclists and pedestrians where it is suitable to go 60kph, maybe even sometimes 70kph. Some of the ones where we have a lot of school children and we have a lot of cyclists and a lot of intersections, like some shopping areas, 30-40kph may be much safer for everyone."
"What we ultimately want is for vehicles to be travelling at safe speeds on all of our roads. What is a safe speed on one road will be different for what is a safe speed on another," he said.
The change is being accompanied by other measures, including a network of fixed speed cameras around the country at accident hotspots.