National is accusing the government of being sneaky by including a provision in its regional fuel tax legislation to increase the tax rate if it wants to.
Legislation enacting Auckland's regional fuel tax is making its way through Parliament and is currently being considered by a select committee.
Meanwhile, Auckland City is already making plans for how it will spend the estimated billion dollars the tax will raise over the next decade.
The tax is capped in the legislation at 10 cents per litre and can only be charged in Auckland, but after 1 January 2021 - councils in other areas could opt to put in place a similar tax.
But National's Transport spokesperson, Jami-Lee Ross, said the legislation also gave the government powers to bump the tax up.
"I think it will be very surprising for New Zealanders and particularly Aucklanders to know that the Minister of Transport is writing a law right now which will give him the power to increase regional fuel taxes past 10 cents a litre."
Mr Ross said National thought it was wrong to have a regional fuel tax at all, but to include the ability to raise it by regulation was just not on.
"The power the that the minister has given his government to increase regional fuel taxes past 10 cents a litre doesn't even need to come back to Parliament, they don't need to pass a new law, they don't need to consult on anything.
"Arbitrarily whenever they want, with no time restriction, the government can pass a regulation which would increase regional fuel taxes past 10 cents a litre."
But the Transport Minister Phil Twyford said the regulation-making power was there for future governments to take advantage of.
"Our government has no plans to do that, but this is a good piece of law, we expect it may well be in place for a long time.
"Some future government down the track, for some future council may want to change the rate - it's not our plan to do that."