'Make the floor the law': Shorten on GST

Federal opposition leader Bill Shorten has asked the prime minister to permanently put the government's proposed GST carve-up in place, rather than merely seek a nod of approval from the state and territories.

"Make the floor the law," Mr Shorten said to reporters in Fremantle on Saturday.

"We would say to Malcolm (Turnbull) ... let's legislate the promises that you're making. We will."

Western Australia has long complained about its share of GST revenue distribution, in place since 2000, which dropped to a record-low of 29.99 cents during the mining boom due to the existing benchmarked system.

In September, Treasurer Scott Morrison will seek agreement with his state and territory counterparts to put a "floor" in place of 70 cents per person per dollar of GST, below which no state's relativity can fall, from 2022/23.

This would rise to 75 cents from 2024/2025.

"We have said we accept the government's proposition," Mr Shorten said.

"Why wouldn't we support it? It is our idea.

"But beyond that, we can pay for our promises to find the extra resources for Western Australia because we're not giving $17 billion away to the big banks. Mr Turnbull has never explained how he pays for his promises."

The GST pool will increase by $7.2 billion from 2021/22 to 2028/29.

States and territories are slated to receive $67.3 billion in GST payments in 2018/19, rising to $112.25 billion in 2028/29.

Mr Shorten joined Labor's Fremantle candidate Josh Wilson to campaign ahead of the WA seat's by-election later this month.