Thousands of lamb-meat lovers may have eaten mutton instead after a mid-North Coast abattoir was caught misbranding sheep carcasses.

Tolsat, based in Frederickton, was fined $66,000 for mislabelling older sheep meat as lamb, a more expensive, "premium" product. This week, they were listed on the NSW Food Authority's "name and shame" register.

"Tolsat was prosecuted for lamb substitution offences and non-compliance with the law over a period between early October 2007 and mid-January 2008," said NSW Food Authority chief executive Polly Bennett.

"Meat substitution laws are in place in NSW for a reason. Flouting them also puts other businesses at a disadvantage for doing the right thing."

More than 3000 sheep, also known as hogget, were purchased in different batches. The abattoir had been servicing shops and wholesalers throughout NSW.

The investigation was part of a statewide audit of lamb identification procedures in 2008 by the Food Authority.

At the abattoir, an inspector noticed problems with how staff were conducting dentition checks – a crucial process to determine the age of an animal by examining its teeth.

In a follow-up investigation, officers found discrepancies in the abattoir livestock and slaughter records.

In December, the NSW Chief Industrial Magistrates Court found Tolsat guilty of 66 charges, including false description and failure to comply with the Meat Food Safety Scheme. The charges had been vigorously defended by the company.

Ron Cullen, chief executive of the Sheepmeat Council of Australia said that in 2011, about 90 per cent of the 115,000 tonnes of mutton produced was exported overseas. In the same year, 400,000 tonnes of lamb was produced, half of which was exported.

"Lamb substitution is therefore a rare phenomenon because we export so much mutton," he said. "Australian consumers can be reasonably confident that if a quality butcher or supermarket say it's lamb, it's lamb."