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.
Bing Boy franchisees at the chain's Northland shopping centre and the CBD's Myer Emporium outlets were found to have underpaid two foreign workers almost $10,000 in just three months.
Acting Fair Work Ombudsman Michael Campbell said the troubling findings revealed in the latest audit underscored the need for the hospitality sector to remain a "priority" industry for the watchdog.
"Clearly, the take-away food sector, an industry comprised largely of small businesses, is grappling with the complexity of the industrial relations system," he said.
The audit of fast-food employers is the third and final "wave" of the agency's three-year hospitality industry campaign. The sweeping investigation unearthed similarly high levels of wage underpayments and industrial non-compliance across pubs, bars, taverns, hotels, restaurants, cafes and catering companies nationally.
Fair Work inspectors said the most common violations by fast-food employers were undercutting minimum hourly wages and denying overtime and weekend penalty rates. 
"Inspectors found some employers providing flat rates of pay for all hours worked, with many advising they had adopted this practice to simplify their payroll processes," Mr Campbell said.
"In nearly one-third of cases, the hourly rate paid was not enough to cover hours attracting penalty rates and loadings, resulting in additional errors."
In   October, a Fairfax Media investigation revealed hundreds of thousands of temporary foreign workers were being illegally exploited across the economy. It showed 80 per cent of foreign language job advertisements were offering wages below legal rates. Many of them were openly advertised as "black jobs".
Chinese student Sunny Liu was one of the workers who spoke out about exploitation and underpayment.
Working in a food court and a restaurant, she has been paid hourly rates less than half the legal minimum wage.
"The first ever job I got in Australia, I was paid about $8 an hour, and then it was raised to $10 an hour," she said.
"The minimum wage than was $16.50 ... but I couldn't find any better jobs. I know there are some students who work for $6 an hour."
Council of Small Business Australia chief executive Peter Strong said hospitality employers often hired vulnerable people such as young workers or people from culturally and linguistically diverse backgrounds.
He said the high contravention rate highlighted the need to assist small businesses in understanding their obligations.
"Most small employers are not experts on workplace relations and are often more focused on having a good business with happy and well performing employees than their compliance obligations," he said.
 "Business people in this sector should know there is help available, including from the Fair Work Ombudsman, who would rather inform and help businesses resolve issues cooperatively, than take them to court."