Duffy pointed to the rising cost of living and the Commerce Commission's supermarket market study as drivers for the decline in trust.
"The Commerce Commission's study into the grocery sector found the duopoly is consistently making profits in excess of what it should," he said.
"Couple that with cost-of-living challenges, and no wonder there has been an erosion of trust in the supermarket industry."
Food prices rose 6.4 per cent in the year to April 2022 and Consumer's research found New Zealanders are preparing themselves to spend more on groceries.
In June 2021 a quarter (24 per cent) expected their groceries to increase. By April 2022, more than 40 per cent (43 per cent) anticipated an uptick in grocery expenditure.
And concern about the price of food has jumped significantly.
Groceries went from the eighth biggest financial concern in June 2021 to third in April 2022, beaten only by rent and mortgage payments.
Duffy said Consumer NZ had also noted that in the last year New Zealanders were incurring an increasing proportion of debt on essential items like groceries, fuel and bills.
Both major supermarket chains have recently taken steps to freeze or drop prices.
Earlier this month Countdown announced it would freeze the prices on 500 items over the winter months.
A week later Foodstuffs which operates Pak'nSave, New World and Four Square said it would slash the price of food by an average of 10 per cent across 110 of its most-shopped grocery items.
Duffy said some of the moves by the supermarkets would be helpful for struggling consumers.
"... however we don't think the supermarket giants should be applauded for their efforts. We should be asking how a lack of competition has been allowed to create this situation in the first place."
Consumer is calling for measures to increase competition in the sector and has launched a petition which asks the Minister of Commerce and Consumer Affairs, David Clark, to go further than the commission's recommendations and bring about meaningful change in the sector.
To date, 74,000 people had signed the petition, and it was aiming for 200,000 signatures, Duffy said.
Emma Wooster, Foodstuffs NZ's co-operative public relations head, said trust was important.
"As two New Zealand co-operatives owned by our 430 store owners throughout New Zealand, we're on the shop floor every day, committed to our local communities and being held accountable by New Zealanders every day for what we do.
"We appreciate the trust New Zealanders have put in us and this is consistently reflected in the feedback we get, directly through our own customer research and through well-established and transparent surveys, like Readers Digest trusted brands and Kantar/Colmar Brunton's RepZ Corporate Reputation Index," she said.
After two years of the pandemic, the co-operative owners' passion to deliver for customers was as strong as ever.
"We're all highly focussed on the latest challenge of providing the very best value to our customers in an environment where household budgets are stretched due to high inflation," Wooster said.
Prices had been more than 110 essential products.
"We're working hard to keep costs under control by buying well and running our business as efficiently as possible, so we're not adding cost," Wooster said.
A Countdown spokesperson said: "We're of course really disappointed to hear this as all of our teamwork incredibly hard every day to give our customers a great experience when they shop with us.
"We know it's tough for many Kiwis, given the current inflationary environment we're in and the pressure this is putting on people's budgets. We understand the frustration at seeing rising prices in supermarkets and elsewhere and we remain very focused on how we can keep food prices as low as possible despite all these rising costs," the Countdown spokesperson said today.
Foodstuffs boss Chris Quinn has hit back at claims the $22 billion industry is making excess profits of $1 million each day as claimed.
"We need to be really clear - we don't make excessive profits, and we don't make an excessive profit of a million dollars a day. The claim that is based on is an incorrect assumption," he told the Herald.
The Government made some moves to clamp down on the sector in the Budget.
It bought in a new law to stop supermarket chains from blocking competitors trying to secure land for new stores.
The Government is also expected to announce plans for a grocery sector regulator soon.
