Opposition Leader Bill Shorten has pre-empted a decision on penalty rates by the Fair Work Commission, declaring the industrial umpire will not cut Sunday penalties.

The Commission is expected to decide whether Sunday penalty rates will be brought in line with Saturday pay for hospitality and retail workers after the election.

Mr Shorten has previously said he would accept the Commission's ruling, but today deflected questions about what he would do if penalties were cut.

"The evidence doesn't support it, it isn't going to happen," he said.

"So when people ask me ... what I'm saying is it's not going to happen."

When pressed on what his party might do if the rates were cut, Mr Shorten posed his own question — on aliens.

Journalist: But what if the Commission though says we should cut penalty rates. Will you accept that decision? Shorten: They aren't going to. Journalist: But what if it does? Shorten: Well, sorry — what if alien life makes contact with Earth?

The Prime Minister has also taken aim at Mr Shorten over his position on penalty rates.

Malcolm Turnbull said the Labor leader had not told voters what his party would do on the issue if elected.

"He tries to suggest that if Fair Work were to reduce penalty rates or change them, he would do something about it without saying what," he said.

Mr Turnbull also emphasised the independence of the commission, saying the Coalition would "respect its decisions".