"Small businesses across Australia are shocked by [this] move," Australian Chamber of Commerce and Industry chief executive James Pearson said.

"Making someone pay twice for the same thing, years after the fact, is not a fair go."

The Opposition is lobbying Centre Alliance and Jacqui Lambie to support a Senate motion that would overturn the casual loading offset regulation, created by former industrial relations minister Kelly O'Dwyer.

The regulation gives employers the ability to offset loadings already paid against any order to backpay entitlements to workers claiming to have been misclassified as casuals.

Employers fear they could be exposed to up to $8 billion worth of backpay claims for annual leave and other entitlements by such workers across the economy, after a sex shop worker became the first retail worker to file a backpay claim in July.

Labor senator Don Farrell gave notice on Wednesday of a motion to disallow the regulation, in a move supported by the Greens.

One Nation's two senators oppose the Labor move, while Tasmanian senator Jacqui Lambie and Centre Alliance's Rex Patrick and Stirling Griff are undecided.

Australian Conservatives' Cory Bernardi has not confirmed his position but he is expected to support the government - which would deprive Labor of the absolute majority it needs to knock out the regulation.

Australian Industry Group chief executive Innes Willox urged crossbench senators to reject the Labor push, saying it was "obviously unfair for an employee who has been engaged as a casual and paid a casual loading to be able to pursue years of back-pay for annual leave".

"The disallowance of this regulation would send a very dangerous signal to the CFMMEU and plaintiff law firms who are currently pursuing 'double-dipping' class actions against employers," Mr Willox said.

The Construction, Forestry, Maritime, Mining and Energy Union's $12 million casual class action against labour hire company WorkPac is listed in the Federal Court on Monday.

It is one of at least seven dollar cases filed in the wake of last year's landmark Skene decision in the Federal Court, which ruled that a truck driver employed casually through WorkPac at a Rio Tinto mine was actually a permanent employee owed benefits.

Labor's industrial relations spokesman Tony Burke said the legal impact of the regulation - and its disallowance - was "limited" but that "the principle here is important."

"Labor will always oppose Liberal attempts to erode the rights of workers," Mr Burke said.

"Casual work is meant to be just that: casual. Using casual contracts for jobs that are in fact permanent is an abuse.

"As the Federal Court found, the 'double dipping' here has been performed by the employer - they're taking advantage of the insecurity of casual work while still getting permanent hours out of their workforce."

Industrial relations minister Christian Porter said Labor must explain to small business owners "why they should be forced to pay twice if a worker has already received a casual loading".

"If they can't do that, then it's clear that swapping from Bill Shorten to Anthony Albanese really hasn't changed a thing," he said.