Hi and welcome to ADVrider.
Questions I would ask are: where about's are you (you might like to fill in your profile) and what sort of riding do you want/plan to do?
A DR has a pretty reliable 200k until reserve, plus 60k's on reserve. In my opinion: If you're sticking to the main roads a stock tank is ok 90% of the time.
But for example if you wanted to head around the east cape on a weekend that's 326 k's and fuel can be patchy if you miss a station or they've run out. An IMS tank will see you all the way around without any problems.
For some of the longer cross country adv riders like clints MMM ride a stock DR tank is a bit on the small side. You can carry a small back-up tank if you need an extra few k's but for piece of mind it's nice to have the extra range.
IMO: The 32 Litre safari tanks are overkill for NZ, and they're bloody cumbersome.