ENFORCEMENT: Loose enforcement of contracts in New Zealand's $7 billion meat industry could be undermining exports and long-term returns to farmers.

Fresh meat on sale in some New Zealand supermarkets could be up to four months old.

Supermarkets are selling some meat as fresh when it may have been in storage for months, using a technique called "gassing".

Advances in technology make it possible to keep the meat looking fresh for longer, but the practice involves pumping oxygen and carbon dioxide into packaging to help meat retain colour and gloss.

Supermarket giant Progressive Enterprises has acknowledged it pumps carbon dioxide and oxygen into some of its meat packs to extend their shelf life. The owners of the Countdown chain say the practice limits bacterial growth.

An investigation by Australia's consumer watchdog magazine Choice found gassing and improvements in refrigeration technology made it possible for meat to be stored for months before it was packaged, posing potential health hazards for customers because harmful bacteria could grow to dangerous levels while the food remained attractive to the eye.

Clostridium Botulinum (botulism) was of most concern because of the severe illness it causes.

The new techniques meant chilled beef mince, for example, could be stored for up to 44 days, and some lamb cuts for up to 16 weeks. Progressive said it was a legitimate way of preserving the quality of the meat and that consumers need not feel duped, or concerned.

Spokesman Luke Schepen said meat prepared away from the supermarket's own on-site butcheries was sometimes treated with oxygen and carbon dioxide, but the majority of meat sold in Progressive stores was either vacuum-packed or sold in standard cling-film packaging.

The country's other major supermarket chain, Foodstuffs, said it did not "gas flush" any meat products. Retail general manager Rob Chemaly said their meat was largely cut and packaged on site by qualified in-store butchers, without anything being added to the packaging.

"However, some pre-packaged meat products were gas-flushed with food-grade oxygen and carbon dioxide by a small number of suppliers."

Suppliers were obliged to adhere to industry food safety requirements and were monitored by the New Zealand Food Safety Authority.

The country's largest independent butchery chain, The Mad Butcher, said it did not use additives of any kind to extend the shelf-life of its meat.

"We are the corner butcher shop just like your granny used to go to, and there are no additives used to extend the shelf life of any meat we sell," said operations manager Dan Adams.

Supermarkets overseas use carbon monoxide to make their meat appear fresher. It reacts with the myoglobin in the meat, producing a healthy-looking red colour, even when the meat is aged. Carbon monoxide is permitted as a food additive in both Australia and New Zealand.

New Plymouth butcher Peter Morrison, who owns the Kiwi Butcher Shop, said supermarkets used such techniques because they increasingly had centralised processing facilities.

Although the meat was safe to eat, he said buyers should know what they're getting.

"I think supermarkets could do more to educate people ... about what's going on to make the meat look so nice."

Green MP and safe food campaigner Sue Kedgley called for more honesty and transparency.

"Consumers have a right to know what's been done to their food."