It's a good question A-Kiwi and I love hearing about folks who are starting out in riding bikes!! As a brit I have no clue which bikes are available in NZ but you can do worse than read into the whole range of stuff that is in the HUBB about, for instance:-
1. bikes for those of short inside leg measurement.
2. bikes for females to ride.
2. low cc bikes (125s, 250s and whatever).

Yeah, I know you have been hanging out here for a while, but there really is a hell of a lot of information/opinions in here.
Reputedly, the HUBB search system is not so good so just look around and use google with key words to suit.

Also, listen to what people are saying in your own home area, including ride as many bikes as you can get your hands on. Borrow them from other riders that you meet with - but make up your own mind and don't be unduly swayed by opinion, even well-informed opinion.

Anyway, after that few pence worth, you could consider the Yamaha "serow" XT225 which is a Japanese bike that is imported to the UK unofficially (so known as a grey import). The latest new versions are 250cc but for years it has been a 225cc single which has been highly regarded here in the UK for its' capability and simplicity, low weight etc, including the low seat height. I know, I own one, but just treat this as one more opinion

ps the serow is neither "plastic" (not too sure what you are thinking there) nor an all-out dirt bike, but it does have offroad capability and is great on gravel roads for instance.

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Dave