Simon Bridges labelled Jacinda Ardern the "fleecer-in-chief" as he blames the Government, not fuel companies, for high petrol prices.
The National Party leader's comments come after a draft report released this morning by the Commerce Commission stated a lack of competition between petrol companies could be contributing to the high petrol prices Kiwis pay at the pump.
Ms Ardern said: "Now we have the Commerce Commission essentially confirming that that is what is happening in New Zealand.
"Over the last decade we have seen those returns climb. I can tell New Zealanders we cannot stand by while they are facing that pressure at the pump and while they are being fleeced.
"It's not right, it's been happening over the last decade."
However, Mr Bridges had a decidedly different view of the matter.
"The biggest fleecer isn't the petrol companies, it's Jacinda Ardern and her Government.
"Jacinda Ardern is the fleecer-in-chief with all of the taxes piled on," he said.
Mr Bridges doubled down on his argument during question time in Parliament this afternoon, where Finance Minister Grant Robertson was answering on behalf of the absent Prime Minister.
"How can she say that Kiwis are being fleeced by petrol companies when her Government has legislated for 24 cents of petrol tax increases since coming to office, when according to MB [Ministry of Business, Innovation and Employment] petrol company margins have increased just 0.1 cents in that time?"
Mr Robertson replied by pointing out the previous government's record on petrol taxes.
"I can say on behalf of the Prime Minister because that is the import of the Commerce Commission report.
"I would note in terms of the rest of the members question that the average increase under National for the fuel excise duty was 5.76 per cent and the average proposed increase from this Government is 5.6 per cent."
The Government increased the petrol excise duty by 3.5 cents per litre in September 2018, and additional 3.5-cent increases were set for 2019 and 2020.
National introduced six excise taxes on fuel between 2008 and 2017, raising the price 17 cents.
In today's draft report, fuel in the regions took a hit, with South Island fuel from major outlets linked to weaker competition and higher prices.
The core problems outlined in the report included that "an active wholesale market for fuel does not exist in New Zealand"; major companies have joint infrastructure networks and supply relationships that give them an advantage over potential rivals; and any new importer faces "significant challenges".
Wholesale prices appeared higher than a competitive market and that flowed through to retail pricing.
The investigation found companies such as Gull and other small retailers "had a positive, but limited impact on retail prices".
It recommended boosting competition by making greater contractual freedoms so resellers could switch between suppliers and enable wider participation in the joint infrastructure.