The parent who takes primary responsibility for the children after a breakup should get a better deal under property relationship laws, the Children's Commissioner says.
Commissioner Andrew Becroft said the Property Relationships Act has failed to put the interests of children first, and he wants coming reforms to ensure that they are not being disadvantaged after relationships break down.
The law is being reviewed by the Law Commission, which is expected to provide its final report to the Government in October.
"The presumption is that responsible, mature adults can be trusted after a fair property split to make proper provision for their children," Becroft told the Herald.
"Experience shows that hasn't happened and children's interest haven't been prioritised.
"This is a once-in-a-lifetime opportunity to rejig the Property Relationships Act so it is made much more child-friendly."
Research has shown that custodial parents - who are typically women - usually suffer financially after a breakup.
Becroft said he wanted the law to recognise and compensate for this, though he did not have a specific solution in mind.
The Commission agrees that the law needs to be more child-focused. It is reluctant to change the equal split of assets after a breakup, saying it would make the process less predictable and lead to more court action and higher costs.
A more discretionary approach is used in Australia, where the primary caregiver can get between 5 to 20 per cent more than their partner.
Becroft agreed that changing the 50-50 split of assets could lead to messier settlements. But he said there could be another way for the economic disparity between separated parents to be addressed, and has urged the Law Commission to consider it.
There are already measures in the law which are designed to protect children after a breakup, including orders which prevent a house from being sold or which grant occupation rights to one partner so the children can remain in the home for a period of time.
However, these orders are rarely used and poorly understood.
In one case, a woman told the commissioner she had sold a jointly-owned house quickly after her breakup even though it would be bad for her children, because she had been wrongly advised she would have to pay rent on it.
Becroft said the courts should use the orders more often, and they needed to be promoted so parents knew they were available.
"What happens if there's a relationship breakup in April, with a mother and three children, and a family house and they're all at primary school?
"There should be the provision, at least, for the mother to have occupation of the house for the benefit of the children until the end of the school year."
Michael Fletcher, a senior research fellow at Victoria University's Institute of Governance and Policy Studies, has investigated how each person in a relationship fared after a breakup in New Zealand. He said women's incomes were on average 19 per cent lower while men's were 15.5 per cent higher.
Women were far more likely to have taken on primary responsibility for the children, he said.
"Unless the way the property gets divvied up offsets that income change, you're at risk of a double whammy.
"The children wind up with the main carer, who is worse off in income terms and worse off in asset terms."
The disadvantage was not short-term, he said, and often lasted for many years after a relationship ended.