The new modelling,based on public service analysis of Australian Bureau of Statistics data,predicts businesses could be forced to pay"casuals"between $18.5 and $39.4 billion for six years of accrued leave if the court rules against them.
It also shows the extent of damages that could be extracted by class actions launched in the wake of the Federal Court's decisions against some businesses that employ large numbers of casuals.
The Attorney-General's modelling looks at the number of casual workers,whether they do regular hours,how long they have been with their employer and what they are paid to arrive at the figures,which dwarf even industry body Ai Group'sestimate of an $8 billion backpay claim.
Even at the lower $18 billion figure,about 540,000 workers would receive backpay,while if the court chose a much more expansive definition about 1.2 million workers could be eligible.
Rossato,the case the High Court is being asked to overturn,concerned a casual mine worker who worked more full time hours on a roster set a year in advance,despite being classified as a casual.
The Federal Court ruled the miner worker's actual patterns of work meant he was actually employed on a permanent basis but his employer,labour hire company WorkPac,is seeking leave to appeal to the High Court and Mr Porter hopes to join that case.
"The economic effect of the decision inRossato is to sanction a process of"double-dipping",which undoubtedly must be a matter of real concern to Australian employers in the current economic climate,"Mr Porter said. Mr Porter,who is also industrial relations minister,wants employers to be able to offset the loading,typically 25 per cent,they pay casuals against any backpay claim.