Motorists outside Auckland may unwittingly pay some of the regional fuel tax intended to help fix the city's transport woes.
The additional 11.5 cents-a-litre is due to be paid by Aucklanders from July, raising $160 million a year.
Auckland mayor Phil Goff repeated his appeal to the parliamentarians assembled in Auckland that, without the extra revenue, the city couldn't tackle its growing congestion.
Aucklanders were prepared to pay their share, via the regional fuel tax, Mr Goff said.
It's likely that from July, for every litre of fuel an Aucklander pumps, the oil company selling it will pay 11.5 cents more in tax.
But whether that money actually came from an Aucklander, rather than someone paying a higher price to the same oil company in another part of the country, was not clear.
The concept is called price-spreading, and it was a recurring theme as the finance and expenditure select committee sat in Auckland to hear views on the tax.
"It's just an illusion to think that this tight ring-fence means that only Aucklanders will pay for it," trucking lobby group Road Transport Forum chief executive Ken Shirley said.
"That's political posturing, it's not commercial reality."
The Automobile Association (AA) said its Auckland members were divided on the tax.
Adviser Mark Stockdale said uncertainty over who really paid more was a problem.
"That undermines completely the regional fuel tax - fatally in our view," he told RNZ.
"Motorists outside Auckland will be deeply frustrated if they believe they are paying a few cents a litre extra in tax, specifically to pay for Auckland projects."
The AA wanted the tax legislation to require the government's transport agency to monitor and report regularly on fuel prices movements.
The only oil company appearing before the committee was Gull,whose retail development manager Karl Mischewski confirmed the tax bill could be funded from anywhere.
"There is a potential for that sort of cross-subsidisation," he said.
Gull's main worry was the administrative cost of tracking where fuel was sold.
Mr Goff believed there was already enough scrutiny of oil pricing.
"The government and parliament itself is a watchdog," Mr Goff said.
"You've seen that down in Otaki where the fuel companies tried to leverage the system to their advantage.
"People will speak out against that and people will want to know, if petrol goes up excessively in other parts of the country, why that's the case."
Auckland Business Forum spokesman Michael Barnett said the council and other local bodies should be selling assets.
Industry group National Road Carriers chief executive David Aitken said the regional tax favoured operators based outside the city, but that drove in or through it.
Mr Aitken would prefer congestion charging for using certain routes inside Auckland.
Hamilton City Council also wanted to join the regional fuel tax party.
Its growth and infrastructure committee chair David MacPherson said the city had to halve its proposed $500 million
spending on growth projects because it could not afford them.
"Parts of Hamilton and the North Waikato are actually growing fast at the moment as the spillover from Auckland happens," he said.
"We're seeing Auckland's problem as not actually an 'Auckland problem' - it's an Auckland-Hamilton growth corridor problem."
Hamilton wanted to join the regional fuel tax club next year, two years earlier than the proposed date for
councils outside Auckland to make the move.
Public consultation is underway in Auckland for another ten days, with the empowering legislation expected
to be passed late next month.