The Commerce Commission will be under "considerable pressure" while undertaking its proposed investigation into petrol companies, its chairman Mark Berry has told MPs.

But the Minister responsible for the ComCom said the regulator was "no stranger to pressure".

Berry told the Economic Development, Science and Innovation select committee this morning that it may take longer than a year to complete a thorough investigation into an uncompetitive market.

Under new legislation – the Commerce Amendment Bill which was passed last month – the Commerce Commission was given the power to initiate market studies into certain industries.

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The first investigation would almost certainly be into petrol markets after Prime Minister Jacinda Ardern expressed her concerns that consumers were "being fleeced" at the pump.

Commerce and Consumer Affairs Minister Kris Faafoi said he would be nominating the petrol markets to be investigated next month.

He said today he expected that study would take a year.

Berry told MPs the ComCom's $1.5 million funding would allow for one market study a year.

But he warned MPs the time pressure could be problematic.

The first market study into petrol companies is expected to begin in December.

Berry said it would not be completed until December 2019, or later.

"I do appreciate that people have a wish to know the results of these sorts of inquiries within much shorter time frames.

"But the experience is that if you put those kinds of time pressures on, the result is you don't get the kind of detailed study that you would have wanted to get."

He cited similar issues the Australia Competition and Consumer Commission (ACCC) had faced in regards to time pressures.

"The ACCC's most recent market inquiries – many of them have been extended over the 12 months to get to the end of doing the kind of detailed analysis they think is necessary," he said.

"While I know it sounds like a long time to wait until December 2019 we will be under considerable pressure to deliver the kind of study people want by that time."

Speaking to media, Berry said the ComCom would endeavour to do the study within 12 months but, in doing so, "the pressure is simply our staff working very long hours and commissioners working long hours to turn that work around."

Asked if he thought the ComCom needed more funding as a result of these pressures, Berry said: "our goal is to do the thorough job within the 12 months. My expectation is that we can do that".

"The pressure is us performing and meeting the deadline".

Asked if he would be willing to give the regulator more time to conduct the study, if it asked, Faafoi said he "didn't want to put the cart before the horse."

"But I think, from the discussions I have had with them, yes it would put them under pressure but it's a pretty important study for us to do, so I think 12 months is alright; we will see how they go."

He said the ComCom "is no stranger to pressure – we think 12 months' is long enough if they came back to us and said we might need more time, we will deal with it."

But he said that was "a hypothetical".