https://www.airalo.com/oceania-esim
This is the e-sim I was looking at and you get a weeks free trial. That’s if your phone takes one. Otherwise in NZ, Skinny is the cheapest at $8 per month but the amount of data is about 250MB per month but you can buy more and top up each day via the app as you need. Skinny,co,nz is the website and look at prepay. You can buy them, from a supermarket or any appliance store that sells laptops and phones.