Staff at a restaurant in Chinatown were paid as little as $5 an hour, slept on milk crates in the kitchen and were sacked via social media, according to a complaint to the workplace watchdog.

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

Hospitality workers' union says staff at Hochi Mama were maltreated.

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

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