A union has alleged to the Fair Work Ombudsman that staff at Chinatown restaurant, Hochi Mama, were underpaid by thousands of dollars and discovered they had been fired after being deleted from a staff online chat group.

Hospitality workers on Tuesday night protested at the city restaurant and its sister city venue Straight Outta Saigon.

The venues have been contacted for comment.

Union Hospo Voice says pay slips show wages of just $21 per hour and no penalty rates, with calculations suggesting one staff member was underpaid almost $10,000 in a just a few months.

It also claims migrant kitchen staff were paid a $70 flat rate for working 12-14 hour shifts, or as little as $5 an hour.

Former Hochi Mama bartender Alex Pugh said he was underpaid almost $4000 for four months' work.

He says he and other staff found out they'd been sacked when they were deleted from the staff's online group chat on WhatsApp.

"It was so disrespectful. Whenever it happened, everyone in the venue could see someone had been fired. It left them all feeling fearful, and wondering who was next?"

But he said the kitchen staff, made up of migrants and international students, were worst affected and paid flat rates of $70 or $100, working double shifts over 13 or 14 hours.

"I saw these workers sleeping on milk crates out the back of the restaurant," he said.

"It's not okay for anyone to be treated like this, particularly in a wealthy country like Australia."

Ben Redford, from United Voice, said hospitality workers are fed up with wage theft and poor treatment.

"What Hospo Voice found at Hochi Mama and Straight Outta Saigon lifts the lid on some of the most shameful practices in hospitality," he said.

"We demand the watchdog condemn this callous method of sacking workers and thoroughly investigate wage theft and exploitation of migrant workers at these venues."

Australian Associated Press