Bill Shorten has promised to "stop the rot" of penalty rate cuts if Labor wins the next election.

Mr Shorten dragged his daughters along for the ride as he campaigned at Luna Park in Melbourne on Saturday.

The opposition leader was there to pay tribute to workers who gave up their weekends and school holidays so others could have fun.

Speaking to a dozen employees before they opened the gates to the famous amusement park, Mr Shorten bemoaned the fact some Australian workers stood to lose hundreds of dollars over the 10-day Easter break.

In an industrial relations rallying cry, he conceded the Luna Park workers were safe from recent penalty rate cuts, thanks to a deal struck between their bosses and union.

"But hundreds of thousands of people have had their penalty rates cut," he said.

Labor has promised to reverse the reductions within 100 days if it wins the May 18 election.

"Everything is going up in Australia except people's wages, and the rot stops when we reverse the cuts to penalty rates," Mr Shorten said.

"We believe that the lowest paid workers in Australia - when they give up their time away from their family, when they serve and make sure the rest of us are having a great time -you shouldn't have to pay for it with a cut to your own pay."

Labor has calculated retail, hospitality and pharmacy workers will lose between $220 and $370 in slashed penalty rates over Easter.

And the Labor leader claims the cuts are only the beginning, warning bigger reductions are in store if the coalition is returned to power.

The Fair Work Commission approved the penalty rate reductions, with further cuts to be phased in from July.

Mr Shorten defended his unusual - but not unprecedented - plan to overturn their decision through legislation.

"Our position is not taken lightly. We respect the independent umpire, but they got this one wrong," he said.

He resisted the urge to climb aboard Luna Park's dodgem cars, conscious of how hooning around and crashing into children might look on the nightly news.

But he might regret taking his youngest daughter for a turn on the carousel.

Mr Shorten copped a barrage of questions about the price of his climate change targets when he fronted the media.

Less than an hour later, the prime minister delighted in Mr Shorten's refusal to provide a figure.

"He's stuck on a costings merry-go-round which he doesn't seem to be able to get off," Scott Morrison told reporters in Sydney.

Meanwhile, Mr Shorten is demanding answers over the coalition's purchases of water under the Murray-Darling Basin plan.

He wants to know if the prime minister is convinced that $80 million worth of water buy-backs are above board.

'Produce all the documents," he said.

"Our river system is stuffed and it is stuffed because this is a government who hasn't had a plan to look after the whole of the river basin."