Under the law, anyone can defend themselves against another person, using force that they believe is appropriate.

That could mean using a weapon - such as a gun, knife or even an umbrella - or fighting an attacker, say criminal defence experts.

Auckland criminal defence lawyer Gary Gotlieb said self-defence was simply proven.

"It could be you're walking down the street and someone comes at you - and you're holding an umbrella - and you instinctively hit them in the eye or any other area to protect yourself ... that's self-defence because you've obviously defended yourself."

Another Auckland criminal defence lawyer, Shane Tait, said people had to prove that the force that they used was appropriate.

"Obviously if a 69-year-old woman is chasing you around the store with a feather, you're not going to go at her with a shotgun and get self-defence," Mr Tait said.

"It's seen as what was reasonable to use - a weapon or fighting back - at the time that an attack happened."

Mr Gotlieb said that in some cases defendants had lost their claim of self-defence.

"It's often cases like at parties where a fight has happened and someone goes and gets a gun or a knife and comes back to join the fight - that is not self-defence."

THE RULES, THE CASES
Crimes Act, Section 48: Defence of oneself in reasonable circumstances. Crimes Act, Section 55: Necessary force to stop someone entering a person's home.

April 2009: The owner of Tokoroa's Aotea Chinese Restaurant and Takeaways, Zhuo Feng Jiang, wrestled a semi-automatic .22 rifle off a masked robber and shot him in the leg. Police decided against charging Mr Jiang.

September 2008: Otara liquor store owner Virender Singh was charged with injuring with intent to injure after he defended himself with a hockey stick against five drunken teenagers, one of whom had stabbed him in the thigh. A justice of the peace dismissed charges.

2006: Greg Carvell shot a machete-wielding Ricky Beckham, 29, in the stomach with a handgun at his father's Penrose gunshop. Police charged him with possession of a pistol for unlawful purposes but in June 2007 the charges were dismissed.