Thousands of pregnant mothers will lose money under a government crackdown on paid parental leave that could start as early as January.
But Labor says it will never votes to tighten access to parental leave payments and it's hopeful Senate crossbenchers will all oppose the move.
Laws to tighten access to the paid leave scheme, set up by the previous Labor government, are before parliament and could start as soon as January, meaning women who are pregnant now could miss out on benefits.
It's estimated 80,000 women each year will be worse off under the changes, which reduce the 18 weeks of leave paid for by the government if an new parent's employer offers paid leave.
"This is a direct attack on paid parental leave, a direct attack on those mothers who are trying to combine their work and family responsibilities and will leave thousands and thousands of new mothers in this country worse off," opposition families spokeswoman Jenny Macklin told reporters in Melbourne on Sunday.
She believed there were between 40,000 and 50,000 women already pregnant who would be left up to $12,000 worse off.
But Social Services Minister Christian Porter said more than half the parents who used the scheme, about 90,000 of them, would not be affected by the changes.
"Currently a parent earning $140,000 annually can receive a combined government and employer PPL amount of more than $44,000 - this is more than another parent working a minimum wage will earn in an entire year and that is not fair," he told AAP in a statement.
Labor says the vast majority of women affected were on low incomes.
"Why should nurses, or shop assistants or other people who have foregone 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?" Opposition Leader Bill Shorten told reporters in Burnie.
Ms Macklin hoped Senate crossbenchers would join Labor in opposing the bill.
"In the past the crossbench have understood just how important it is for women to have time at home with their newborn baby," she said.
