Australia has the highest proportion of temporary labour in the OECD at one in four.
Many casual workers aren't getting paid much more than their permanent counterparts, and some are even making less, new research has found.
The peak body for unions claims the research has "blown apart the myth" from the business lobby that casuals are paid a significant premium for the loss of leave rights and job security.
A casual loading, usually of 25 per cent, is provided for in many awards and agreements but an Australian Council of Trade Unions paper says many casuals aren't receiving that premium.
The ACTU used Australian Bureau of Statistics data analysed by Joshua Healy, a senior research fellow at the University of Melbourne's Centre for Workplace Leadership.
He compared 2016 data on median hourly wages for workers based on ordinary earnings and hours of work in 10 occupations which have the highest casual density and account for half of all employees in the classification.
In most of these occupations, there is a casual wage premium of about four to five per cent.
While school teachers have an average 22 per cent premium, of the other nine occupations six had a premium of two to five per cent and three got paid between three and six per cent less than their permanent counterparts.
Australian Industry Group chief executive Innes Willox says employers are required to pay casuals 25 per cent more than the award rate.
He said the fact an average permanent employee was paid relatively more than casuals when the loading was removed was not surprising.
"Many permanent employees have lengthier service and higher skills than many casual employees," Mr Willox said.
Jobs with casual premiums of five per cent or less included labourers, cleaners, laundry workers, sales assistants, personal carers and assistants, hospitality workers and sales assistants.
Casuals with negative wage premiums worked as clerks, sport and fitness staff and packers and assemblers.
ACTU secretary Sally McManus said while some people choose casual work because they need flexibility, many preferred the paid leave and security of permanent work.
“This research shows what people who are being ripped off in casual work already knew – that our work rules are unfair and we need to change them," Ms McManus said.
“Casual pay on average is actually around two to five percent more in similar occupations, and many people get paid less than permanent staff, particularly in lower-paid work – not the supposed 25 percent premium."
The ACTU's paper says Australia has the highest proportion of temporary labour among Organisation for Economic Co-operation and Development nations.
Mr Willox argued casual employment rates had remained at the same level for the past 20 years, and different definitions of casual employment made it hard to compare countries.
"The ACTU's ongoing attempts to demonise casual employment need to be rejected by all political parties," he said.