A bill that empowers the Commerce Commission to investigate anti-competitive behaviour has passed its third reading in Parliament, allowing a study into the fuel market to start by the end of the year.

The Commerce Amendment Bill, which passed with the support of all parties with the exception of Act, will mean the commission can carry out a market study if it - or a minister - deems it in the public interest.

Prime Minister Jacinda Ardern has prioritised the bill in the past two weeks and has already lined up petrol companies as the first sector to be looked at.

The bill was about "encouraging an honest business environment" to promote competition and protect consumers, said Minister Carmel Sepuloni, speaking in Parliament during the third reading on behalf of Commerce and Consumer Affairs Minister Kris Faafoi today.

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The commission had been allocated $1.5 million a year to carry out market studies, she said.

The first market study will be announced in December.

The bill hit the headlines late last month after Ardern said that petrol companies appeared to be "fleecing" motorists.

National opposed the bill at its first reading, but has since supported it after lobbying from the Prime Minister.

She said cross-party support "will send a very strong message that we're serious about these issues in New Zealand".

National supported the bill despite wanting ministerial oversight, which could have considered the cost to businesses.

"An open-ended power can be incredibly expensive for an industry ... and the cost always ends up being paid by the consumer," National MP Judith Collins said at the third reading.

The amendment gives the Commerce Commission the power to conduct market studies into certain sectors to gain a better understanding of how the market functions.

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A minister needs to nominate a sector they believe to be uncompetitive before the commission investigates – Ardern said she would be nominating the petrol industry first.

In a press conference last month, she said consumers were being "fleeced" at the petrol pump and pointed the finger at the fuel companies for their high margins.

The Greens have also called for supermarkets to be investigated by the Commerce Commission.

"For far too long, the supermarket duopoly between Foodstuffs and Woolworths has hurt producers, growers and small business in New Zealand," Green Party MP Gareth Hughes said.

"[Supermarkets] should absolutely be investigated by the Commerce Commission following an investigation of the petrol industry; this would benefit both suppliers and consumers."

Ardern has said she wouldn't be surprised if industries such as supermarkets faced the same scrutiny as the petrol companies at some point.