The competition watchdog, the Commerce Commission, is indicating its hands are tied over grocery prices.
An Australian study using OECD figures found the price of food in New Zealand has gone up more than 42% since 2000, the second highest increase among OECD countries.
South Korea had the highest food price hikes in the period, of 48.2% followed by New Zealand (42.5%) and Australia (41.3%), according to figures analysed by a University of New South Wales academic.
Japan recorded only a 1% increase in the period.
The commission says that in declining an application by Foodstuffs or Woolworths to acquire The Warehouse chain of stores, it effectively allowed a third competitor to attempt entry to the "highly concentrated" groceries market.
But it says a third player has not eventuated.
It says the commission's ability to look into price control issues typically applies to natural monopoly situations, and not a duopoly.
Produce rises the reason - Minister
The Government says the increase is due mainly to the rise in the cost of fruit and vegetables.
Consumer Affairs Minister Heather Roy says while more competition would drive prices down, fruit and vegetable costs are pushed up by other factors including the higher cost of fuel and adverse weather conditions.
And she says consumer preference for organic produce means the average price of food is higher.
Consumer New Zealand chief executive Sue Chetwin says there is little doubt a supermarket duopoly is behind the price rises, and all consumers can do is to shop around.
She says this country has a "double whammy" in that much of the food produced here is exported, so New Zealanders end up paying export prices.
She says it would be difficult to add competition to the supermarket industry.
Supermarket 'would welcome competition'
Foodstuffs managing director Tony Carter told Morning Report his company would welcome further competition.
He says The Warehouse made an attempt to break the duopoly, but did not try to have the lowest prices.
Foodstuffs, the operator of the Pak'n'Save and New World supermarkets, is in competition with Progressive Enterprises which runs the Woolworths, Countdown and Foodtown stores.
More competition urged
Associate professor from the University of New South Wales School of Business, Frank Zumbo, who studied the OECD data, says Australia and New Zealand both have supermarket duopolies that act as "cosy clubs" and that's the reason for the high prices.
Professor Zumbo told Checkpoint it's a fundamental economic principle that more independent competition leads to lower food prices.
He is calling for stronger enforcement from New Zealand and Australia's anti-competition watchdogs.
The Australian Government has stressed it is moving to open food retailing to extra competition.
The ABC reports the figures are based on an OECD food price index and show the change in prices from a base year, not the current comparative level of prices across countries.
The ABC says the OECD average was 33%, or 27.5% when the OECD's high inflation countries were excluded.