Labour is challenging the government's figures on the cost of extending paid parental leave from 14 to 26 weeks.
On Wednesday night parliament passed a bill to extend it to 26 weeks over three years, but the government is going to use its financial veto to kill the legislation.
It says the cost would be $150 million a year, which can't be afforded.
Labour MP Sue Moroney, who drafted the bill, told parliament on Thursday she had documents from the Department of Labour showing it would cost $285.6 million over three years.
Finance Minister Bill English didn't say that was wrong.
"The simple point is that the proposition would double the cost of paid parental leave," he said.
"I look forward to the day when the government has those sorts of choices - we don't have those choices now."
Mr English wouldn't give a commitment to increase paid parental leave in 2015 - the year the government expects to have its budget back in surplus.
"That will be one of a long list of aspirations New Zealanders have, and at the time it may not be the most important," he said.
Mr English defended the support the government gives to families.
He said paid parental leave currently cost $154m a year and had increased by $67 a week since 2008, early childhood education subsidies cost $1.4 billion and Working for Families $2.6b.
Ms Moroney's bill has strong public support and on Thursday a new lobby group was launched.
The 26 for Babies coalition was unveiled at parliament, attended by 26 babies and toddlers.
Spokeswoman Deborah Morris-Travers says every parent and grandparent knows that 14 weeks is very early to be sending babies "out into the world".
The bill has the backing of all the parties in parliament except National and ACT.
The public now has a chance to make submissions on it to a select committee, and it will go through the legislative process to its third reading before Mr English invokes the financial veto.
Governments can use the veto to block legislation which would cost money that isn't in the budget.