National Party leader Simon Bridges is refusing to confirm whether he'd overturn a new petrol excise tax, which comes into force in July, if he becomes Prime Minister.
The first legislation passed after the Wellbeing Budget was released on Thursday was to increase the petrol excise tax by 3.5 cents per litre.
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The Excise and Excise-equivalent Duties Table (Budget Measures - Motor Spirits) Amendment Bill passed under urgency on Thursday and will come into force on July 1, just as road-user charges are also increased.
Transport Minister Phil Twyford said the excise increase "is absolutely necessary to overcome the infrastructure deficit that we inherited as a result of the unbalanced approach under the previous government's transport policy".
The alternative, he told Newshub, would be "gridlock in our cities, lost productivity in the regions, and more deaths on our roads.''
National has criticised the increase, the second of three annual hikes. It comes as prices at the pump continue to creep up.
Transport spokesperson Paul Goldsmith said last week that the increases "swamp any benefit to be gained from the potential hand-outs of the Budget".
But Bridges refused to say he would overturn the new excise tax if he came into power next year.
Asked by The AM Show host Duncan Garner if he would overturn the tax, Bridges confirmed "we are certainly not going to have any more", but would only say the party would "consider" pulling back this Government's increases.
"Right now, in the Transport Budget, they have dramatic underspends. I wouldn't put the [taxes] on, my inclination would be to not have them, but if you are already there and they are already there, there is no way I am putting more on."
He defended his position to wait and see by saying he didn't know what state the economy would be in if National did win Government - meaning whether he would need the extra cash.
"I don't know what sort of a shambles I am inheriting should I be lucky enough to be Prime Minister in 18 months time.
"I would need to responsibly consider [the state of the economy] if I had the privilege to be Prime Minister."
The legislation passed in Parliament will give the Government an extra $363 million worth of fuel taxes over two years, Goldsmith said. That's calculated by applying this year's excise tax increase and next year's one, he said.
On Friday, Twyford said the Opposition's resistance to the excise tax increase "lacks credibility and integrity". He said the previous government raised it while in power - "and now they oppose this regular increment?"
Last year, the Government announced the first Commerce Commission market study would be into the retail fuel market.
Prime Minister Jacinda Ardern said the public was being "fleeced" by fuel companies and excise taxes were only a small percentage of rising fuel prices.
Consumer Affairs Minister Kris Faafoi said at the time that it was in the public interest to ensure people weren't paying too much for fuel.
"There are existing indications of competition problems in the retail fuel market that are of concern to me, such as the more-than-doubling of petrol and diesel importer margins over the past decade.
"It's also a market that is hugely important to consumers and to our economy, given the extent to which we rely on fuel and the size of the market, with around 6 billion litres of petrol and diesel consumed for land transport use annually."