Coles and Woolworths meat price wars are taking such a massive toll on butcher shops that some butchers are resorting to cheating to keep their prices down, even when it's downright dangerous for customers.

Some are swapping quality cuts for cheap, chewy meats.

Other culprits are using an illegal chemical - sulphur dioxide, or SO2 - a preservative that’s strictly controlled because it can cause severe, even fatal reactions in people allergic to it.

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Butcher Craig Cook says most in the industry wouldn't do it, but a few butcheries have been busted for adding sulphur dioxide to inferior meat.

“The product that they're working with is not fresh and they've got to add something to it to make it last,” Cook said.

“Mince that’s been made the previous day, has lost its colour, or gone off in colour and turns quite brown. There's one way to fix it - throw it out and make it fresh, or add preservative to it and re-mince it through, and all of a sudden it will look like it's been made off fresh meat again.”

Sulphur dioxide is not allowed at all in mince and whole cuts of meats. Tiny amounts are allowed in sausages and certain deli meats like salami, devon and frankfurts, but some butchers have been caught adding huge amounts.

NSW Food Authority's Enforcement Manager Christine Tumney says inspectors are constantly on the prowl for offenders.

“It can bring on an asthma attack if you're susceptible to that, it can also have an allergic reaction. So you can find there's respiratory issues, or other sorts of allergic conditions,” Tumney said.

The Authority tests mince in the shops, and samples of suspect sausages are lab-tested for sulphur dioxide levels.

“The truth of the matter is good quality lamb is still very expensive, so they're looking for an alternative that’s cheaper.”

Some meat suppliers have also been fined this year for illegal meat substitutions.

There's a huge difference between genuine lamb that’s killed at up to twelve months old; hogget that’s one to two years; and mutton that’s two to ten.

Lamb is around $20 a kilo, while hogget and mutton is around $10 a kilo - just half the price.

“The first thing to look for is the visual difference,” Cook explained. “But the real telling point is the smell - you can really smell hogget.”

“I'd have to say 99 per cent of out butchers are really doing the right thing, and really endeavoring to go without any of these cheating issues and so on, and they don't want to use them, but we've got an odd few bad eggs that do.”