Auckland's mayor has fired back at National for its claims the council has $300 million of unspent fuel tax money just sitting in its accounts.

The party is campaigning on scrapping the region's fuel tax within its first 100 days in Government, if elected.

Finance spokeswoman Nicola Willis says the revenue it has already made will continue to be available for transport projects.

But mayor Wayne Brown says that money is already committed.

"Just cause it's not in our bank- it's like if you build a house that's going to cost you $1 million, you make sure you've got $1 million before you start. As it gets built, you slowly pay for it."

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