Nationals leader Warren Truss says details of Tony Abbott's paid parental leave scheme have not been finalised, revealing the government is talking to rural groups about how to ensure farmers can receive the payment.
But Labor leader Bill Shorten and Greens leader Christine Milne have questioned whether the scheme will even be put to a vote in the Senate.
The admission from the Nationals leader comes amid a revolt on the Coalition back bench over the $5.5 billion paid parental leave scheme, with some Liberal and Nationals senators – including John Williams, Barry O'Sullivan and Ron Boswell – publicly voicing concern and threatening to cross the floor.
On Wednesday, Mr Truss said the government was in talks with the Nationals Farmers Federation and rural women's groups over the scheme.
“We are talking with the National Farmers Federation and rural women's groups as to how to best design the scheme to ensure that farmers in this sort of circumstances are in fact eligible," he told ABC Rural.
“The test is not so much about income as about the fact that they have been actively involved in farming operations and therefore they are breaking in their employment.”
One of the major concerns flagged by rural MPs over the scheme is that it may advantage working women in cities over farmers' wives who may be performing unpaid work on family farms.
In response to Mr Truss's comments on Wednesday, Labor leader Bill Shorten said the scheme was not in the "design phase" but in the "crazy investors phase".
"It isn't going to see the light of day, I don't believe," he told reporters in Sydney.
"The paid parental leave scheme, no one wants it, Tony Abbott."
Greens leader Christine Milne appeared confused by the Acting Prime Minister's remarks about working on the design of the scheme.
"What does that mean?" she said in Sydney.
"Exactly what does the Coalition intend to put forward to the Parliament?"
Earlier on Wednesday, Senator Milne told Radio National that it looked like Mr Abbott would not have a chance to introduce his "signature" policy to Parliament, noting that there was no legislation yet for the scheme.
"I'm not going to waste any brain power on this until the Prime Minister actually releases the draft legislation."
On Monday, Fairfax Media revealed a compromise proposal put forward by Senator Williams that would extend the current Labor scheme - which pays the minimum wage for 18 weeks - to 26 weeks. The scheme proposed by Senator Williams would also include superannuation payments.
Mr Abbott's proposal pays women their full replacement wage for 26 weeks, capped at a maximum payment of $50,000 and includes superannuation.
Palmer United Party leader Clive Palmer, who controls four senate votes, indicated on Tuesday he could back that plan if women who were not working also received the payment.
And on Wednesday, the Department of Social Services released a review into Labor's less generous PPL scheme, which showed it had a “clear effect” of delaying mothers' return to work up to about six months after their baby was born.
By 18 weeks following the birth, 85 per cent of post-PPL mothers had not returned to work. This compares with 78 per cent of mothers before the PPL was introduced.
At 12 months, there was also a slight increase in the post-PPL group returning to work. Seventy three per cent of women in the post-PPL group had returned to work, compared with 69 per cent of the pre-PPL group.
The review, which was required under the 2010 Paid Parental Leave Act, found that existing PPL arrangements had a “particularly strong impact” on delaying the return to work of mothers who were self-employed.
It also found that after a year, mothers on lower incomes were seven percentage points more likely to return to work a year after the birth of their baby.
Labor's PPL scheme was introduced in January 2011 and provides 18 weeks pay at the minimum wage to the primary carer of a new baby.
The review, which received 65 public submissions, found that, "many stakeholders called for the payment duration to be extended from 18 to 26 weeks. A few of the submissions that dealt with duration included a proposal that the payment duration be extended to one year."
It also found that the PPL scheme had been implemented “on schedule” and claims and payments were “largely implemented as intended”.