If your main problem is the rentals being booked up, you're willing to spend/lose some money and you have enough to buy a bike then why not buy one on arrival and sell it when you leave? It is a fair bit of hassle for two weeks ownership but it's definitely doable.
If you haven't discovered TradeMe yet then have a look on there, it's the primary selling place for used motorcycles in NZ. Get in contact with a seller before you arrive, exchange money for motorcycle shortly after arriving, ride motorcycle, list for sale at $1 reserve (you'll get market value on the day), exchange motorcycle for money, leave NZ.
Failing that, any dealer should be able to give you an indication of what a bike they're selling might be worth to them in a couple of weeks with an extra 2-5000km on it. It'll be a lot less than you what paid for it to start with but as you're aware, renting for 2 weeks is pretty expensive anyway.
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