Laws to limit access to the paid parental leave scheme are before parliament and could start as soon as January.
That means women who are pregnant now could miss out on benefits.
Labor says tens of thousands of women will be worse off each year under the changes, which reduce the 18 weeks of leave paid for by the government, if a new parent's employer offers paid leave.
Labor leader Bill Shorten says this is unacceptable.
"Why should nurses or shop assistants or other people who have forgone pay rises in lieu of getting a paid parental benefit now be slugged because they've negotiated these conditions, and not get the minimum paid parental leave."
The Coalition says the move is to prevent pregnant women from claiming entitlements from their employers and the government.
But Mr Shorten says it will only do harm.
"Is this government really saying that the only way they can sort out the economic mess which they've taken Australia into is, on one hand slugging, the working conditions of working mums, or instead they say at the same time they've got enough money to give millionaires tax cuts and to give multinationals and the big banks massive tax cuts?"
Opposition families spokeswoman Jenny Macklin says she believes there are between 40 and 50 thousand women - already pregnant - who'd be left up to $12,000 worse off.
Ms Macklin says it will force them to make difficult choices.
"Either they will have to cut short their paid parental leave to go back to work because they have to pay their bills, and that will of course mean they will spend less time with their babies. Or they decide to stay at home and that means they'll be thousands of dollars worse off."
Social Services Minister Christian Porter says more than half the parents who use the scheme, about 90,000 of them, would not be affected by the changes.
Mr Porter says a parent earning $140,000 annually can receive a combined government and employer Paid Parental Leave amount of more than $44,000.
He says this is more than another parent - working a minimum wage - will earn in an entire year and that's not fair.
But (Ms) Jo Briskey from advocacy group, Parenthood, has told Sky News, the government's proposal will put undue pressure on families.
"This is going to force more women back to work far sooner than they had hoped for, far sooner than they were ready for. It's going to increase the financial pressure on families, especially as they try and find the childcare they need if they have to go back to work sooner than they had planned. There's nothing fair about these changes, these cuts to paid parental leave."
Labor says it hopes Senate crossbenchers will oppose the move.
