A new law recommended by the Law Commission would recognise family violence victims who kill are usually reacting to years of mounting abuse.
Under the current law, a person must be facing an immediate threat to use self-defence - but in a report released today, the commission proposes widening the defence in family violence cases.
Daryl Kirk, who shot dead her mother's abusive partner in Lower Hutt last year, is awaiting sentencing next month after being convicted of manslaughter.
Her lawyer unsuccessfully argued self-defence, saying she was confronted by a drug-crazed Adam Watkins, armed with a machete.
Law Commissioner Wayne Mapp said under the proposed change to New Zealand's self-defence law, someone like Kirk could have been acquitted.
The current law failed to recognise that family violence victims who killed, were usually reacting to years of mounting abuse and often didn't see any other way to stop it, he said.
Self-defence could even be argued in cases of pre-meditated killing, such as a woman killing her abusive partner while he slept.
"Such a claim would only be successful if existing in the context of a sustained long period of family violence, and that if you didn't do that, then the next thing that was going to happen to you was that you were going to be killed instead."
A small subset of family violence homicides - less than 10 percent, or less than 5 percent of all homicides in New Zealand - happen when a victim of family violence kills their abuser.
Of 24 cases examined by the Law Commission in which victims of family violence killed their abusers from 2001 to 2015, self-defence was claimed in 10 of the 16 cases that went to trial.
It was successful in three cases, and failed in seven.
Women's Refuge said New Zealand lagged behind Britain and many Australian states, which have amended their self-defence laws to recognise the situation of battered women and other victims of family violence.
Its chief executive, Ang Jury, said the Commission's most important recommendation was continued education for judges, lawyers and police about family violence.
"Domestic violence can be likened to a kind of domestic terrorism lasting for years and a woman who has been controlled and violated over a long period of time will sometimes react in a way very different from that of someone not experiencing the same sort of routine violence and cruelty," she said.
"Having these circumstances better understood by judges, lawyers, police and the wider public could, if combined with these types of legislative reforms, go a long way to helping prevent future injustices."
Justice Minister Amy Adams said the Law Commission had rightly identified that many family violence victims felt trapped.
But it would be a significant change to make self-defence available almost at any time in certain situations, she said.
"In Australia they have said that the threat to a person's life does not have to be imminent, but it does have to be inevitable.
"And I personally think that that provides some stronger safe-guards than the way the Law Commission has phrased it, which is simply that it doesn't have to be imminent. I think you do need some significant checks and balances in place."
However, the Law Commission maintains its recommendation would remove current inconsistencies.
A senior legal and policy adviser, Nichola Lambie, said it would not turn self-defence into a get-out-of-jail free card for pre-meditated murder.
"We're not recommending changing the test for self defence, which is that the response must be reasonable in all circumstances as the defendant saw them. That's a matter for the jury. What we're trying to do is remove the focus on the imminent circumstances."
Other recommendations in the report include:
- A change to the Evidence Act 2006 allowing a broader range of evidence about incidents of family violence to be provided to support a claim of self-defence.
- Continuing education of judges, crown prosecutors and defence lawyers to improve understanding of family violence within the criminal justice system.
- Ministry of Justice to undertake further work to address the three strikes system as this may have unintended consequences for victims of family violence who kill their abusers. The report suggests the Ministry consider how the law could be changed to allow appropriate sentencing in deserving cases.