Biodegradable bags should not replace ordinary plastic shopping bags because they do not break down properly in landfills, a waste consultancy company has said.
Bags made from vegetable starch and other biodegradable substances break down in compost bins without leaving harmful substances behind.
But Jo Knight, director of Zero Waste New Zealand, said they did not break down properly in landfills, where they were kept out of the weather.
Friends of the Earth co-director Bob Tait agreed, saying society had gone too far using disposable products already. "The most sensible thing is to use reusable bags."
He said a lot of biodegradable packaging was corn-based, which displaced crops from land that would otherwise grow food.
Starch-based packaging "mucked up" recycling schemes if it was thrown in with plastic, he said. Several readers emailed the Herald yesterday, asking why supermarkets did not use biodegradable bags.
Alex Williams brought several biodegradable supermarket shopping bags home with him after a holiday to England and found they crumbled within two months in a hot car.
"That would be good for the ones [bags] you see on beaches or floating in the harbour," he said.
Ms Knight said biodegradable bags caused less of a litter problem than plastic. But they could cause problems if they were mixed up with plastic bags sent for recycling.
She said the best option was to remember to take reusable bags to the supermarket and recycle plastics.
Reader Miranda Verswijvelen, originally from Belgium, said Belgian shoppers quickly learned to take reusable bags after plastic bags were banned from supermarkets there.
"People bring crates or reusable bags ... [If you forget] you just have to throw everything in the boot of your car," she said.
Ms Knight said biodegradable packaging was a good option for anything that touched food as long as it was composted afterwards.
A third option - degradable plastic bags with a chemical additive that helps them break down into small pieces - did not solve the problem because the plastic was still left behind.
Polyethylene shopping bags make up just 0.2 per cent of the rubbish to landfills but remain for decades because they do not break down.