Women deserved better protection from the law, he said.
"And we actually need a better way, I think, of dealing with these issues and give them protection in the short term and over a medium period of time. It's a challenging issue."
A decision is pending on potential changes after nearly 500 detailed submissions from individuals and groups were received after the Government released a discussion document last August.
Justice Minister Amy Adams in March released a summary of submissions, and said, overall, people were broadly supportive of the ideas for change.
New Zealand has the highest reported rate of intimate partner violence in the developed world, and ideas floated to address that are wide-ranging and, in many cases, radical. They include:
• Create stand-alone family violence offences. This could include an offence of psychological violence or an offence of repeat family violence offending.
Another idea is to allow a judge to take into account the seriousness of harm as part of a pattern of behaviour.
New Zealand judges take into account a person's previous convictions, but this is viewed as an aggravating factor and is not explicitly aimed at family violence.
• Form an "additional pathway" for victims, perpetrators and family who want to help stop family violence, but do not want to go through the court system.
• Set out in law options for police when responding to family violence. For example, in Victoria, Australia, police must choose whether to take criminal action, take civil action such as applying for a protection order, or make a referral to services.
The discussion paper also proposed "empowering police or an approved non-government organisation or iwi service provider to apply for a protection order on a victim's behalf" - for example when a victim is too scared of a perpetrator to apply for an order herself.
Police officers can already initiate a protection order, for example when a police safety order was breached. But the new proposal could enable women's refuges to initiate protection orders.
The paper also suggests that when a protection order is breached police might be either required to arrest in all cases or required to take some action - either arrest, issuing a police safety order or referral to an agency such as an anti-violence course.
• Update the legal definition of domestic violence, with one idea to more clearly explain the concept of "coercive control". This would mean opportunities for authorities to intervene are less likely to be missed, because the significance of incidents are not underestimated.
• Give clearer direction to courts to consider the potential for parenting arrangements to expose a child or adult to further violence. The risk of violence is known to be elevated when a child is handed over to their other parent, for example.
• Make it easier for the sharing of information between the courts, police and the agencies and community organisations that deal with families. Currently, the Privacy Act can affect the ability to pick up on family violence.
Meanwhile, an open letter to Key from 33 groups, including the Salvation Army, has called on the Government to raise the age at which offenders are treated as adults from 17 to 18.
In April, an independent panel recommended the age be raised to 18.
Key said Cabinet had not yet considered the matter, but would look at it.
Children's Commissioner and former principal youth court judge Andrew Becroft told Radio New Zealand that the age should have been raised long ago.
He said New Zealand was out of step with similar countries and the situation was "unacceptable".
"It represents really an enduring stain on our otherwise good justice record."
Tonight, Key said there had not yet been enough of a discussion to get a sense of how different ministers felt about changing the age.
"I think there will be some that will think it is a good idea, and some that might [question that]."
Helen Clark's Labour Government introduced a bill to raise the age for leaving state care and youth justice to 18 in 2008 but the bill was dropped by Key's incoming National Government at the end of that year.
Adams said last year that she had asked officials to look again at the issue, but her predecessor Judith Collins - now back in Cabinet as Police and Corrections Minister - was firmly against it.
New Zealand First's social development spokesman Darroch Ball today called on the adult justice system age to remain at 17.
"The National Party must get tough on law and order. Our communities have had a gutsful of weak-kneed liberal alternative pathways for criminal youth."
