Deputy Prime Minister Winston Peters says if petrol companies don't start acting responsibly, the Government could pass a law preventing them from "ripping off New Zealanders".

And he has taken aim at the banking sector, which the Government has told to get its act together, and even the menswear industry for fleecing consumers.

Yesterday the Government confirmed that the retail fuel market would be the first Commerce Commission market study, following the passing of a law last month giving it the power to initiate market studies.

Prime Minister Jacinda Ardern said that the commission, which would report back in 12 months, would provide peace of mind for petrol buyers.

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But Peters ramped up the attack on petrol companies this morning, noting that the margins for fuel importers more than doubled from 7 per cent in 2008 to 16 per cent in 2017.

"If you look at the margins that were being charged eight [to 10] years ago and what's being charged now, they're up dramatically," Peters told Newstalk ZB.

"What's the excuse? There is none."

He said the Government, following the commission's investigation, would do one of two things.

"See whether they can understand sweet reason and play the game, or pass a law to ensure they can't go on fleecing New Zealand customers."

The law would stop petrol companies "ripping New Zealanders off", but he declined to say what an acceptable margin would be.

"If you aren't prepared to pass a law, your commitments aren't worth the air they pass on. You have to have the preparedness to make a stand in the interests of the consumer."

Peters then ripped into the banking sector, noting an investigation in Australia that has found widespread systemic issues and has led to a banking levy expected to raise about A$1.5b annually.

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Last month in New Zealand, the Financial Markets Authority and the Reserve Bank released a report into the culture of 11 New Zealand banks and found that they did not manage poor staff conduct well, and were slow to remove perverse sales incentives.

The four major banks in New Zealand - ANZ NZ, ASB, BNZ - are Australia owned, and Peters said it was barely credible for New Zealand banks to present an image of purity while their Australian owners were being irresponsible.

"We are asked to believe in New Zealand the same children of this offshore Aussie parent aren't acting like Ned Kelly, but no, here they're as pure as the driven snow.

"It's time for us to look at these matters with eyes wide open."

Asked if the retail menswear industry was ripping off customers, Peters told Newstalk ZB's Mike Hosking : "I think it is, actually, frankly a lot of it's rubbish, as you know. Not the stuff that you're buying, but poor old Joe Bloggs out there, he's being ripped off."

He added that he was not advocating new laws to regulate the banking or menswear sectors, though the Government has said it would look at regulatory options if banks did not pick up their game.

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Peters referred to former US President Theodore Roosevelt's saying to "speak softly and carry a big stick".

"Sometimes in terms of business, you have to do that," Peters said, to provoke a "come to Jesus" moment to make them "behave responsibly".

"I'm an unashamed responsible capitalist. Why can't we all understand the need for the Government to act in the interests of its citizens?"

Later today, Peters is joining Ardern in Auckland for a bilateral meeting with Korean President Moon Jae-in.

Peters said New Zealand had a good relationship with Korea, but it was timely to improve it given the challenges to denuclearise North Korea.

He said the chances of that were improving and was "better than it has been for a long time, ever since the 1950s".

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Asked if Trump deserved credit for that, Peters said: "As Ronald Reagan said, you can't argue with success."