The gas gives the impression that the fish is fresh when it actually isn't.
According to Food Standards Australia New Zealand fresh and frozen fish is being imported into Australia from Asia with an undisclosed foreign ingredient - carbon monoxide.
"It's treated with a gas from a cylinder and then it's wrapped, quite often frozen and imported to Australia," spokesperson for Food Standards Australia New Zealand, Lorraine Belanger said.More stories from Today Tonight
Chances are you've unknowingly eaten fish treated with the chemical, which makes it look fresher than it really is.
"This has the potential to mislead some consumers because the fish might be older than it is implied by its colour," Belanger said.
100 tonnes of tuna treated with carbon monoxide are imported each month from South East Asia - it's currently perfectly legal but that could soon change.
In 1995 the European Union banned carbon monoxide and now Food Standards Australia New Zealand wants to follow suit. They're preparing a report which aims to ban the import of fish which is processed using the so called 'flushing method'.
"Chemical changes can occur in the fish that might create a food safety issue," Belanger said.
The concern is that people will eat off fish, which could lead to food poisoning.
Children, pregnant women, the elderly and those with compromised immune systems are the most at risk.
Chris Gambrellis has sold flushed fish to Melbourne restaurants and hotels for the past four years. He says "the carbon monoxide does give it a good appearance, it's a bright colour. We have comments people refer to it as fluorescent. We get a longer shelf life out of it."
Gambrellis says "the ones that use the flushing technique (are) the barramundi, the yellowfin tuna, the swordfish, the marlin, mahi mahi."
Australians have an insatiable appetite for seafood. Each year we consume 193,000 tonnes of it and 72 per cent of that is imported. Thailand, New Zealand, Vietnam and China are our biggest providers.
"The benefits are the costing, the shelf life as well, and supply - consistent supply," Gambrellis said.
Despite all this and the cheaper prices, fishmonger Angelo Christopoulos says he always chooses Aussie produce over imports.
"The majority of people who come here want Australian produce," Christopoulos said.
Food Standards Australia says its proposal to ban the use of carbon monoxide flushing in fish should be complete by the end of this year.
This reporter is on Twitter at @tinekae