Unions have slammed the Federal Government's decision to join a casual workers court case.

The court case is seeking to stop casual workers “double dipping” on leave entitlements, claiming leave on top of their casual pay loading.

Jobs Minister Kelly O'Dwyer says the current rules are causing anxiety for small businesses.

“It's generally one or the other, but not both, and the certainty needs to be made very clear,” Ms O'Dwyer said.

“That's why the Commonwealth is intervening in this particular case, small businesses need that certainty.

“Each and every day they face decisions about whether or not they employ people, and that is somebody's future.”

Labour hire firm WorkPac brought the case to the Federal Court, after it had earlier ruled against the company in a similar dispute and allowed a casual truck driver to claim leave entitlements.

“This is a new test case particularly looking at the offset arrangements,” Ms O'Dwyer said.

“I am intervening in this case to clarify what was not clarified in the previous court decision.

“The anxiety that this has created among small businesses right across the country is one that needs to be settled and settled very quickly.”

Australian Council of Trade Unions (ACTU) President Michele O'Neil said the truck driver in the first case had been working for the company for two-and-a-half years, and had been getting paid 30 per cent less than permanent workers.

“What the company did was, instead of employing him as a permanent worker as they should have, they used a labour hire firm to bring him in on a rate that was 30 per cent less,” Ms O'Neil said.

“So how is that worker doing anything other than ensuring that his rights are protected? And that's what the court has done.

“The court has looked at the circumstances and said; ‘This isn't casual, this is fake casual and he's entitled to be paid his accrued annual leave’.

“The Minister doesn't understand the law, this is not that unusual a decision.

“There's been more than 20 years of precedent cases like this where the courts have said if you enter into a sham, if you have someone that's employed long term, if you have someone that's on a regular shift, regular hours, then you can't just call that person a casual and get away with it.

“So for the Minister to suggest, and for the business lobby to suggest that somehow this is an unusual case that has put at risk the whole employment of casuals is wrong.”