Tuna sold in NZ may appear fresh due to carbon monoxide

Questions are being raised over the freshness of the tuna we're buying.

What may look fresh could have been treated with carbon monoxide and frozen for months.

Harbourside head chef Shaun Tyagi says the fresher the better.

"We're using a bluefin tuna today, it's coming from Hauraki Gulf," he says. "Fresh tuna is amazing in texture, amazing in taste and you can tell the difference right away."

But not all tuna you buy may be as fresh as you think.

Some of the fish has been treated with carbon monoxide - an odourless, flavourless smoke process, which prevents oxidisation. 

Instead of turning brown, the fish retains its fresh pink-coloured flesh, even after freezing and thawing.

Seafood New Zealand is concerned consumers could be misled.

"In order to import carbon monoxide-treated tuna into New Zealand, it needs to be labelled," says Seafood New Zealand seafood standards manager, Cathy Webb.

"There is a potential to be misled if that labelling is not sold alongside of the product,"

Food Standards Australia New Zealand says it's safe to eat.

"There is currently no evidence of a food safety concern with the use of smoking, including the use of odourless flavourless smoke, to process fish," a spokesperson says.

While carbon monoxide is not permitted to fix or alter the colour of the fish, it says "this prohibition does not extend to carbon monoxide that is naturally occurring in smoke".

Seafood New Zealand says it's a loophole it wants ended.

"It fixes the fresh red colour and that has the potential to mask signs of age or spoilage - and that's where our concern lies with the treatment," says Ms Webb.

So if you want fresh and can't tell by looking at it, ask when you buy it or check the label.