The head of Fair Trade in New Zealand says the supermarket duopoly held by Progressive Enterprises and Foodstuffs has stymied the adoption of ethical certification in this country.
Adoption of the Fair Trade mark by producers has been relatively slow in New Zealand, lagging behind more established markets in Britain and Europe.
The organisation says only 2% of all coffee sold in New Zealand is Fair Trade, compared with a quarter in Britain.
Sales have grown rapidly since it was introduced here in 2004, but the total amount is only $17.5 million.
Executive director of Fair Trade in New Zealand and Australia, Steve Knapp, believes supermarkets are to blame.
He says UK supermarkets competed to try and be the most "fair trade retailer", which increased sales in the UK.
But Mr Knapp says there's not the same sense of competition in New Zealand, because both supermarket chains already have around 50% of the retail market.
Despite this, Mr Knapp forecasts sales of Fair Trade products here and across the Tasman to double to $100 million next year.
He says that will be largely driven by a recent decision by Cadbury to make all its dairy milk chocolate Fair Trade, which will result in it sourcing 4000 tonnes of cocoa from Ghana next year, putting more than $1 million into the community there.
Meanwhile, Mr Knapp says the BNZ has become the first major company in New Zealand to be certified as a Fair Trade Workplace, which means all its tea and coffee is Fair Trade.
He hopes this will encourage other employers to follow suit.