Almost half of all fast-food outlets audited by Australia's workplace watchdog are underpaying their staff, and the overwhelming majority violate payroll obligations.

The Fair Work Ombudsman's nationwide investigation into the fast-food sector has alarmingly revealed just 53 per cent of 565 spot-checked employers have been paying their staff correctly.

The findings add to concerns about exploitation and illegal wages in the Australian hospitality industry, where a where a black economy of cash-in-hand jobs appears to be deep-rooted.

In one recent case, the operators of popular Asian take-away store Bing Boy told authorities that wages of $10 an hour were "common" and $13 an hour was considered "good pay" for hospitality staff in Australia.