Greens Senator Sarah Hanson-Young has joined deputy Liberal leader Julie Bishop in stumbling over superannuation policies.
Senator Hanson-Young told ABC radio on Wednesday that the minor party would seek to tax superannuation at the same rate as income.
"In order to make it fair, you should be paying the same tax rate as you do for what you're earning on your super. Why not? Why shouldn't it be the same?" she said.
Labor MP Mark Butler, who was appearing on the same program, labelled Senator Hanson-Young's comments "extraordinary".
"I'm not quite sure that Sarah has quite outlined the Greens' party approach to superannuation because it would essentially undo 30 years of retirement incomes policy," he said.
Senator Hanson-Young later corrected her comment.
"I've had a text from my chief of staff sitting outside, sending me the table," she said.
"It's basically, as I was saying, a progressive tax rate."
Ms Bishop had a self-confessed "gotcha moment" on Melbourne radio on Tuesday when she was unable to say how the coalition's super transition-to-retirement changes worked.
The Turnbull government faces pressure to overhaul part of its plan to wind back tax breaks on superannuation.
Labor and coalition members fear that despite government claims the budget measures will only impact on the rich, there is evidence they will also hit middle-income earners.
West Australian Liberal MP Ian Goodenough said he had received "a volume of correspondence from constituents objecting to the changes".
But the dilemma was finding other areas of revenue if the changes are dropped.
Opposition superannuation spokesman Jim Chalmers said the budget was in "tatters" just a month after it was delivered.
He had concerns the changes to the "transition to retirement" scheme - designed to save the budget $640 million over four years - would impact on people "up and down the income stream".
Under TTR, workers can minimise their tax by tipping additional money from their salary into superannuation via salary sacrifice, then taking some of the money back as retirement income from the super fund.
Because super is taxed at a concessional rate, it means they pay less income tax.
"The evidence that we've been receiving about the impact of the transition to retirement changes is that it will impact on people on middle incomes for example and not just people at the very top," Mr Chalmers told reporters on Wednesday.
Labor will formally respond to the changes before election day, but Mr Chalmers said any reform of the superannuation system needed to be "careful and considered".
"People do have cause to be concerned about their retirement incomes," Mr Chalmers said.
It is understood the May budget superannuation changes did not go to a coalition party room meeting for approval.
However, if the Turnbull government is returned, the draft legislation will go through a coalition backbench committee before being discussed and endorsed by the party room.
Prime Minister Malcolm Turnbull told reporters in Brisbane the budget, including the superannuation changes, had the "full support naturally of the government, the cabinet and the party room".
However, he admitted it could impact on some middle income earners, whose funds in TTR would be taxed at 15 per cent rather than being tax-free.
"It is an increase in taxation but it is still a very concessional rate of tax ... but what we are doing is making the system fairer," he said.
Assistant Minister to the Treasurer Alex Hawke said the transition scheme was a "very niche issue", affecting about 115,000 Australians, and the changes were modest.
He talked down reports of a revolt among coalition MPs.
"There's been plenty of discussions inside the coalition about every element of our changes," he told Sky News.
Cabinet secretary Arthur Sinodinos said "not all individual measures" were discussed in the coalition party room as part of the budget discussion before the budget was handed down last month.
"If we win the election, there will be consultation on various changes and then legislation presented to the party room," Senator Sinodinos told Sky News on Wednesday.
He said it was about ensuring all parts of the community did their share to repair the budget.
