Pressure is mounting on the Coalition to fix legal uncertainty over "double dipping" of casual loadings and entitlements after a major union launched a $12 million class action.

The Construction, Foresty, Maritime, Mining and Energy union case - filed in the Federal Court on Tuesday - brings to seven the number of class actions seeking back pay for workers who claim to have been misclassified as casual.

Industrial relations minister Christian Porter is reviewing the nation's workplace laws. Credit:Alex Ellinghausen

Business leaders are urging Industrial Relations Minister Christian Porter to draft new laws to ensure employers cannot be forced to pay twice for the same entitlement, fearing the trend could leave them exposed to claims for leave entitlements worth up to $8 billion across the economy.

A sex shop worker last month became the first retail worker to pursue backpay, claiming he was wrongly classified as a casual by the owner of the Pleasure Lounge on Sydney's Oxford St, seeking $41,100 plus superannuation.