Auckland Transport estimates it could cost up to $25 million to meet the government's new speed rule to increase speed limits on roads where it has been lowered and put in variable school speed limits.

The Minister of Transport signed the speed rule into law last month, ordering speed limits be reversed to their pre-2019 levels by 1 July next year, and introducing variable speed limits around schools a year after that.

General manager of transport safety Teresa Burnett told the Auckland Transport board on Tuesday that speed limits on 1800 local roads will need to be increased.

The majority of those - 1760 - are in neighbourhoods with schools in the area, where limits were lowered from 50km/hr to 30km/hr.

Auckland Transport estimates that reversing speed limits could cost $7.5m - made up of changing signs and road markings in reduced speed areas.

Setting up variable speed limits around schools, which would lower speeds only during pick-up and drop-off times, is estimated to cost $16.7m.

Another $450,000 will need to be spent on advertising, informing people of the speed changes.

Burnett told the board that no new funding was allocated from the government to cover reversing speed limits, and the cost will have to be covered by Auckland Transport.

Board member and councillor for the North Shore Chris Darby asked if there was any ability to refuse to reverse speed limits.

The board was told it was not a "lawful option" available to Auckland Transport, and there was no room for consultation on whether to carry out speed limit increases.

But, under the new speed rule legislation, there was an option to set new speed limits of 40km /hr in town centres, but that would have to be applied for after 1 July next year.

Burnett noted it would be logistically difficult to change speed limit signs and road markings by 1 July 2025.

Guidance on managing speeds under the new rule from NZTA is still to come.