Bill Shorten insists trade unions and Labor are one in backing the independent umpire, rather than new laws, to decide on penalty rates.
The Greens have called for the incoming government to pass laws protecting penalty rates, as the Fair Work Commission considers a case brought by the retail and hospitality sectors to ease weekend rates.
Prime Minister Malcolm Turnbull says his government would respect the commission's finding, but Labor and the unions are divided on the issue under pressure from the Greens.
Mr Shorten said 61 coalition MPs were on record arguing for the scrapping of penalty rates, while Labor had made a submission to the commission in favour of keeping them.
"I don't accept the assumption that trade unions don't support the independent umpire," the opposition leader told reporters in Adelaide on Tuesday.
Legislation would give the Liberals "the mechanism to be able to chop the independent umpire".
Nationals leader Barnaby Joyce said he agreed with the need to abide by the commission.
"If it just becomes party political decisions you would have pieces of legislation putting wages up, pieces of legislation putting wages down," Mr Joyce told 2GB radio.
"The employer would just say: `Listen, I don't think I should employ you because I haven't got a clue where this is all going to end up'."
The Australian Manufacturing Workers Union - which was reported as backing the Greens idea - said in a statement it did not support laws to take away the commission's ability to decide on penalty rates.
"In the event the Fair Work Commission, or a future Turnbull government removed penalty rates, the AMWU would not rule out any action to protect living standards, including legislative, legal and industrial action," the union said.
Mr Shorten told 5AA radio he believed the status quo would stand when the commission brought down its decision after the election.
"We are going to win the argument," Mr Shorten said.
Workplace law expert Professor Andrew Stewart said a direction made by Mr Shorten when he was a minister had ensured the commission needed to have regard to anti-social or unusual hours when making its decisions.
"If someone was trying to get rid of penalty rates that would make it more difficult," Prof Stewart said.
There was a "very real chance" the commission would make changes to penalty rates, but those changes could see rates increased for some industries.
Any changes would come with safeguards and be phased in to ensure businesses are not unduly burdened, he predicted.
