Petrol prices will go up 11.5c in Auckland on Sunday, as the regional fuel tax comes into force.
There were queues at stations across the city on Saturday, as motorists filled up for $1.99 for perhaps the last time in a while.
Jon Reeves from the Public Transport Users Association says the extra cost - estimated to be about $5 a week for the average Aucklander - people will start tossing up whether to leave the car at home.
"This price increase... may just well tip them over into public transport," he told Newshub.
"Auckland Transport will then think, 'Right - we may have to add some extra buses on certain services."
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The revenue raised will pay for just a fraction of the $28 billion plan to fix the city's transport issues. But Mr Reeves fears it'll be wasted.
"One of [the plans] is building a slow tram to the airport. We'd rather see a fast train built there, which can be implemented in four years and cost a fraction of the cost of a slow tram."
Instead, he says the Government should be making it cheaper for people to catch buses, trains and ferries.
"We would like to see [Transport Minister] Phil Twyford and [Auckland Mayor] Phil Goff both get really serious about this and say, 'Hey - we're going to reduce the cost of public transport."
Mr Reeves says the fares should go down by 25 percent.
Finance Minister Grant Robertson said Sunday's income boost from the Families Package for low- and middle-income families will more than offset the extra they'll be paying at the pump.
"The amount of money we're talking about here on average for a family... is about $75 per week they'll be better off. The increase in petrol is estimated to be around $5, so there's a huge difference."