In the Federal Magistrates Court in Melbourne on Tuesday, the fast food chain admitted to underpaying 693 of its Tasmanian employees a total of $665,695 between 2006 and 2008.

Magistrate Philip Burchardt described the underpayment as enormous and fined Hungry Jack's $100,500.

"More than 30 employees were underpaid more than $4000 each and the highest individual underpayment was over $10,000," he said.

Fair Work Ombudsman director Michael Campbell says the scale of the underpayment to young workers was a key reason the Ombudsman decided to prosecute. "This penalty sends a clear message that the underpayment of young and vulnerable workers will not be tolerated," Mr Campbell said.

"We expect large corporations to exercise their workplace responsibilities, particularly when they employ large numbers of vulnerable workers who are unlikely to know or exercise their workplace rights."

A spokesman for the company said that it unintentionally contravened the Workplace Relations Act 1996.

A manager at one of two Launceston stores contacted last night declined to comment.

The stores involved were Launceston, Mowbray, Devonport, Glenorchy, Hobart and South Hobart.