The government is leaving the door open to making paid parental leave more flexible, but won't support an opposition proposal to do so this evening.
Labour is currently pushing through a bill under urgency to extend leave from 18 weeks to 22 in July next year, then up to 26 in 2020.
National supports the legislation, but also plans to put forward an amendment suggesting that both parents be allowed to take leave together.
Workplace Relations Minister Iain Lees-Galloway said while the idea appeared at first glance to be a good one, his officials had advised him it could actually be detrimental.
"Two parents could have 13 weeks of paid leave together rather than 26 total."
He said that could reduce the amount of time the primary caregiver spent with the baby, which ran counter to the whole point of extending leave.
"That bonding, that attachment between the child and primary caregiver, is extremely important in the child's development."
National has called Labour "dogmatic" in its refusal and said its amendment would simply allow parents more options and flexibility.
Mr Lees-Galloway said the government had not even had the chance to see the amendment National was preparing to put forward, but he accepted there was a "legitimate conversation" to be had further down the track to fully think through the consequences.
"I am absolutely keeping the door open to a conversation with the National Party around these issues," he said.
"It's possible - it's likely that we'll come back and look at this again in the future."
Mr Lees-Galloway said Labour would support a second amendment from National to extend so-called "keeping-in-touch hours" which allow parents to return to work occasionally without losing their benefit.
"We are open to good ideas from the National Party," Mr Lees-Galloway said.
The proposed law is expected to open up for potential changes in Parliament this evening, before its third and final reading later this week.