Are you conversant with gun laws in NZ? A friend there says he may own a gun, but cannot use it in self-defense. That sounds crazy.
Reasonably conversant - and it comes down to some fine distinctions.
New Zealand has the same common law we have in Australia. And the common law of self defence says that a person may use reasonable force in self defence. And if the person is in fear or their life or of serious injury, any level of force is reasonable, including deadly force, and the use of any weapon.
So if you have a gun, and you find yourself in a situation where you honestly believe you (or another innocent person) is at risk of death or serious injury, you may certainly use the gun.
Because that is common law (that dates back centuries) most Commonwealth countries (like the UK, Australia, and New Zealand) do not expressly have laws that say you can use a gun for self defence - because they are redundant. Common law already gives you that right. That distinction is used to say that a person does not have an automatic right to carry a weapon for self defence - basically the argument is "just because you have the right to self defence, that doesn't mean we have to let you carry a gun" but if you do have one, you can use it if you need to.
You'd better be ready though to stand up in court and explain to the jury why you felt the situation was dangerous enough to use that gun. And you can't use it in a less serious situation (I could not shoot somebody who was robbing my house unless I could also explain why I felt I was in danger - if I did feel in danger, I could shoot them, but I would do it because of the threat, not the robbery (the level of force allowed to simply stop a crime is lower).
It sounds more complicated than it is in practice. In most cases, if a person shoots somebody in self defence they are in the clear. But you do get occasional cases like that of Tony Martin in the UK, where he was found to have committed a crime - mainly because he agreed during his interview with police that he didn't feel under threat (which is why you don't answer questions until you have a lawyer next to you) and because the perpetrator was running away.