ACT says plans to stop cars going to the top of Maungawhau, or Mount Eden, in Auckland is undemocratic.
The party's MP, David Seymour, says the proposal has come out of the blue.
He says the Maunga Authority's intention comes after the Government had promised that the authority would not compromise existing public access and rights.
Mr Seymour says ratepayers who fund the maintenance of Auckland's volcanic cones deserve to have a say on decision-making.
A final decision on the plan will be made in April by the Maunga Authority, a Crown, council and iwi body which oversees the maunga (mountain).
Meanwhile, the chief executive of New Zealand Maori Tourism has a message for anyone against the plan to ban cars.
Pania Tyson-Nathan says people need to have a better understanding of what is being proposed.
Ms Tyson-Nathan says New Zealanders have no issues with respecting the position of the Greeks with the Parthenon and other iconic sites around the world, and questions why there would be an issue in Aotearoa.