Labor leader Anthony Albanese says he's waiting for modelling on the latter stages of the government's tax plan, as the finance minister continues to pressure the opposition into supporting the package.
While Labor supports the first tranche of the tax package - geared toward low and middle-income earners - it's not keen on the government proposal to flatten the tax structure in 2022 and 2024 under the plan's second and third stages.
Mr Albanese says he doesn't know what impact the third stage will have on the budget for various income groups.
"We are still waiting for that information from the government," he told Nine's Today program on Friday.
"We will make a sober assessment based upon the facts. What we don't do also, is disregard the impact on the economy."
Finance Minister Mathias Cormann is again warning against any attempt to split the package.
"Any attempt by the Senate to split the package would be rejected by the government in the House of Representatives," he wrote in an opinion piece in The Australian on Friday.
"So the way for Labor to help deliver income tax relief for low-income earners quickly is by voting for our entire plan both in the house and in the Senate in the first week of July."
Not flattening the tax structure will see more Australians pushed into higher income tax brackets due to inflation, he added.
Senator Cormann has been locked in negotiations with the Senate crossbench over the 10-year income tax relief plan.
One Nation is demanding action on power prices and water storage in return for its support, while the Centre Alliance wants more help for pensioners.
Without Labor's support, the Morrison government needs support from four of six crossbenchers to legislate the cuts when parliament resumes in three weeks.