Labor has criticised proposed legislation ensuring Western Australia's GST share stays above 75 cents in the dollar.
Opposition Leader Bill Shorten in August pledged a unity ticket with the coalition on the issue.
But after Labor was sent the proposed legislation, shadow treasurer Chris Bowen expressed deep concern it had failed to guarantee no state would be worse off.
"As we review the legislation, we will be seeking answers about why this guarantee has been excluded," Mr Bowen said.
Prime Minister Scott Morrison says the proposal is expected to be introduced when parliament reconvenes in a fortnight.
"Labor should support the plan. They said they were on a unity ticket with the plan," Mr Morrison said.
State and territory finance ministers will also discuss the draft laws when they meet later this week.
The existing GST system has a lag effect, which meant WA's share plunged to a record low of 29.99 cents for every dollar collected after the mining boom.
It has since increased to 34 cents for every GST dollar collected, while the government also gave the state $1.4 billion in top-up payments.
Liberal governments in NSW and Tasmania have made support for changes to GST distribution conditional on their states being better off.
