Prime Minister Jacinda Ardern says she wouldn't be surprised if supermarkets are next industry under anti-competition scope

Prime Minister Jacinda Ardern says she "wouldn't be surprised" if industries like supermarkets are subjected to increased Government scrutiny under a new law first aimed at petrol companies.

Ardern promised on Monday to rush through a law that allows the Commerce Commission to compel companies to provide data that proves the market they are in is operating competitively.

She said she would nominate the petrol sector as the first market to look into, but the law would allow for more than just one industry to be studied.

LAWRENCE SMITH/Stuff Prime Minister Jacinda Ardern questions petrol company's pricing tactics, while rejecting calls to lower fuel tax.

The genesis of the law lies in a market study that the prior Government attempted when two petrol companies refused to hand over some information. The new law would legally compel them to.

Ardern was asked on Wednesday whether the supermarket sector - which is dominated by two huge companies who own New World, Pak'n'Save, Four Square, and Countdown - could be next.

"Other areas around the cost of living have been raised," Ardern said.

"Particularly when New Zealanders compare what happens in Australia, Kiwis say, 'Why are we seeing that marked increase here?'. I wouldn't be surprised if we see a nomination in other areas like that."

The process would require consultation with the sector and a "nomination" to Commerce Minister Kris Faafoi.

STUFF Prime Minister Jacinda Ardern will nominate petrol companies for the first market study, but the law will likely be used elsewhere.

The New Zealand Food Grocery Council, who represent food and grocery suppliers, was very supportive of the new law in Select Committee, and were keen to focus the Government's eyes on supermarkets specifically.

"In the past decade we have observed an increased international focus on supermarket buyer power, but this issue and its potential impacts are yet to be studied in New Zealand," the group wrote in their submission.

The group were keen to see market studies carried out so the Commerce Commission could identify and address "embedded and potentially harmful market conditions".

DAVID WALKER/STUFF Two companies dominate the supermarket sector in New Zealand.

Retailer group Retail NZ disagreed with the proposed law change, saying the current law was "working well".

"Being the subject of a competition study is expensive, time-consuming and distracting. As a result, being subject to a competition study creates costs for business, which ultimately will be passed on to consumers," the group wrote in their submission the bill.

The duopoly in New Zealand supermarkets has long been the subject of political concern.

Regional Economic Development Minister Shane Jones railed against Countdown when he was an opposition Labour MP, saying the chain were intimidating their suppliers.

Countdown "categorically" denied these accusations.

A Foodstuffs spokesman said the company had no comment on the possibility of a market study.