Australia's biggest business and employer groups cancelled a high-powered phone hook-up planned for Tuesday night to discuss how to counter a Labor-union political campaign opposing cuts to Sunday penalty rates.

While they are divided on how to challenge the campaign, Opposition Leader Bill Shorten has written to every member of the crossbench to pressure them to support Labor's bill to stop a cut to Sunday penalty rates.

In Parliament on Tuesday, Malcolm Turnbull and Mr Shorten traded blows again over the issue, with the Prime Minister pointing out the review had begun under Labor.

He also accused the Opposition Leader of hypocrisy for previously defending the independence of the Fair Work Commission, but now proposing legislation that would override the Commission's decision.