Finance Minister Mathias Cormann has been unable to tell a Senate hearing where a new analysis of Labor's personal income tax plan came from.
But he then tried to turn the tables by asking Labor senators if the data is wrong.
A report in The Australian newspaper on Wednesday, using the "government's analysis" of the tax policies of the coalition and Labor, claims average wage earners will be paying between $500 and $2000 a year more personal income tax by 2024/25 under the federal opposition's plan.
Senator Cormann tabled data that forms the basis of the article at a Senate estimates hearing on Wednesday, which shows the tax people would pay in various occupations under both schemes.
Treasury officials at the hearing said they hadn't compiled the report and the minister couldn't say who had, only that a media adviser had given it to him on the way to the hearing.
He took the question on notice.
Labor senator Kristina Keneally was aghast by Senator Cormann's unusual lack of knowledge.
"You came here and tabled a document that you say is Labor policy, you can't tell us where it came from, you can't tell us who did the analysis, you can't answer the most basic questions, it just got handed to you this morning," Senator Keneally said.
Senator Cormann asked whether she disputed the data.
"Tell me one number that is in there that is wrong?" the minister asked.
Senator Keneally was surprised by the question.
"You say this represents Labor policy but you can't even tell us where it has come from," she responded.
"I don't know where it has come from and you don't know where it has come from."
The government's seven-year, three-part tax plan featured in this month's federal budget.
Labor only supports proposals that start on July 1 - a new low- and middle-income tax offset, which will provide $530 relief each year and lift the 32.5 per cent tax bracket from $87,000 to $90,000 - but is against further bracket changes in 2022 and the removal of the 37 per cent tax rate in 2024.
In government, Labor would almost double the new low- and middle-income tax offset.