PM should 'change tune' if Labor wins WA

Federal Opposition Leader Bill Shorten insists the battle line of the WA election will be drawn on penalty rates, warning the prime minister to change his tune on the issue if Labor wins.

"If the result goes strongly to Labor in WA, Malcolm Turnbull needs to change his tune," Mr Shorten told reporters in Perth on Saturday.

Mr Shorten has tried to paint the state election as a test on changes to penalty rates, following the controversial Fair Work Commission decision to align Sunday rates with lower Saturday rates in the hospitality and retail sectors.

WA's Liberal Premier Colin Barnett has backed the decision to slash penalty rates, promising to match the decision for state-based awards if re-elected.