Rebecca Yi Jeong Shin, who owns Sushi Bay Belconnen, admitted underpaying 22 employees a total of $18,671 over five months between November 2015 and March 2016.

Most of the staff were Korean nationals on working holiday or student visas, and four were still teens, aged between 17 and 19.

The underpayments at Sushi Bay were uncovered as part of an audit blitz in 2016 on more than 40 sushi outlets across Canberra, Queensland and NSW.

Nearly $30,000 in unpaid wages were recovered from stores in the ACT, where five of the seven businesses investigated were deemed to have breached at least one workplace law.

At Sushi Bay, individual workers were underpaid amounts ranging up to $1992, including both weekday and penalty rates, leave entitlements and casual loading. Further breaches in part-time agreements and record keeping were also discovered.

Ms Shin runs a chain of Australian sushi restaurants but allegations of underpayment have dogged her ACT company since 2007. Having previously been put on notice to fix issues, she was formally cautioned by the regulator in May 2015.

Federal Circuit Court judge Brana Obradovic called her actions deliberate, involving "basic and fundamental entitlement[s]".

Ms Shin was fined $20,736 by the court and her company hit with a further $103,680 penalty.

Alex White at UnionsACT said the fine demonstrated the vast scale of underpayment in the capital.

"There is was a wage-theft crisis in Canberra," he said. "A growing number of adult employers [are deciding] to deliberately steal wages from vulnerable young people."

More than half of workers aged under 25 had been underpaid in the past 12 months, according to UnionsACT's latest annual survey.

Fair Work Ombudsman Sandra Parker said addressing the exploitation of vulnerable workers in restaurants and cafes was a key priority for the year ahead.

"Employers who do this will get caught," she said.