United Petroleum has come under fire after a Fair Work investigation found 40% of the franchises it examined were underpaying staff.
The company - which has a station in Kempsey - found to have withheld more than $12,000 dollars in wages.
The Ombudsman will not disclose the locations of the service centres it surveyed, but notes in all but two cases, owners say they'll pay the money back.
The owners of a Sydney and Adelaide station ordered to cough up cash, though.
The Ombudsman notes the majority of the unpaid employees were migrant workers and unaware of their workplace rights.