I can't comment on specifics for NZ (but one month? I thought normal was two weeks notice). But, if the normal notice is one month, and they are asking for more

I can't comment on specifics for NZ (but one month? I thought normal was two weeks notice). But, if the normal notice is one month, and they are asking for more, then explain it's not possible. Moreover, explain that while you are there, you won't be able to provide your best work, and moreover, it might even be detrimental to their business if you are working poorly. Especially if you are the only developer, you could be creating a situation that takes a new dev longer to fix, than for them to do the correct thing from scratch. Read your contract carefully. Also read the laws carefully. You might well have the right to quit when you want. And in NZ I would assume that you would have a strong worker's advocate if the business wants to be shit and not pay you for work done. Also, as others have said, be sure to use any leave you have coming.