Two petrol companies - Gull and Mobil - did not provide all the information the government's fuel price inquiry asked for, Energy Minister Judith Collins says.
But Gull's boss says it simply did not have the data in time.
A government-commissioned report released yesterday said profit margins for retail petrol had increased significantly over the last five years.
But it stopped short of saying the prices were unreasonable, because the petrol companies that took part in the study did not provide enough information.
Petrol prices were especially high in the South Island and Wellington, with gross margins more than 30 percent higher than in other parts of the country.
Ms Collins told Morning Report prices fell about 10 cents a litre in the three weeks leading up to the report, roughly a $300 million-a-year gain for the nation's motorists.
She said two of the fuel companies had not provided all the information that was asked for.
"The companies all agreed to provide us with all the information that we have indicated. Two of the companies, Z [Energy] and BP, provided everything that was asked for.
"Gull and Mobil provided us most of what was asked for, but not some of the essential information that the people who wrote the report believed that they needed to have.
"We did not have the power to compel, we could have gone down other routes where there was that power, it would have been a lot more expensive, a lot more time involved and wouldn't have - more than likely - come up with any better response than we've got now."
Gull general manager Dave Bodger said the company was "about providing value to the motorists" and provided all the information that they could considering the number of staff they had.
"We've got a total of six people in our accounting team ... We provided the information that we could and were able to in the timeframe.
"That's part of the reason, part of the reason too the inquiry asked us ... for certain types of information, we haven't had to have that to run our business and run it well and give value to the motorist.
"I don't think we're the sinners out of this inquiry. If you look at where the competition is it's where we operate."
He said it was a voluntary inquiry, and Gull had participated where they could. He could not recall what the information was that the company had not provided.
Mobil said in a statement it "actively engaged" to the "highest level possible" with the Ministry of Business, Innovation and Employment and Ms Collins.
It said it provided "accurate, relevant and meaningful" information.
Labour calls for Commerce Commission follow-up
The Labour Party wants the Commerce Commission to further investigate whether there is enough competition among petrol retailers.
Labour energy spokesperson Stuart Nash said the government should act swiftly to determine whether motorists were paying too much.
Mr Nash said if fuel companies would not act, extra regulation would be needed to bring fuel prices down.
"It appears that the big boys just aren't listening to what the market is saying, i.e. get some competition, modify your ways.
"If that is the case then the only tool that government actually has is legislation or regulation."
However, Ms Collins said she wanted to hear from the petrol retailers first before deciding whether more regulation was needed to lower prices, or whether the companies could do it themselves.
"When, for instance, we find Gull can't - and other independents can't - get into the South Island or Wellington because of the lack of terminal facilities, that can be dealt with.
"But if the industry comes up with some solutions themselves I want to listen to that.
"This is not short-term stuff, it's been around for quite some time," she said.
Z Energy insists the petrol market is competitive and that its economic returns are reasonable and in line with domestic and international peers.
Mobil said it was reviewing the report's recommendations and welcomed further consultation.
RNZ News has also asked BP for comment.
The government report recommends the Commerce Commission investigate the matter further.
The government has recently given the commission powers that would enable it to compel companies to hand over more information, which could help make things clearer.