Hi Jen,
It depends on where you go, what time of year and so on. Some shops are notioriously bad payers - in fact, some expect their instructors to work for free!!! Go figure!
It also depends on whether you are working full time or are a part time/casual instructor. I've only ever worked as the latter - generally the pay is comparable, but the conditions aren't always as good as a casual. It's rare to get the courses that run on liveaboard trips, for example.....!
Typically, a casual instructor will earn somewhere between NZ$100-150 per day. Everywhere I have worked uses either a daily rate or a course rate, rather than hourly. For some courses, the hours in a day aren't enough!! On a big OW course in summer, I would typically start work at 6am getting the boat ready, split my 8 students into two groups, do 2 dives with each group, drive home, wash the boat etc. I'd get home around 8pm. $100 / 14 hours = $7 p/h, less than the minimum wage.
Some shops now pay a course fee and a per-student top up, e.g. $200 for an OW course plus $50 per student. This is a bit more fair than a daily rate for large courses.
Commission??? As a casual instructor, not a chance. Even for full timers that would be rare..... also don't expect tips (if you work for a charter), NZ is even less of a tipping culture than Aus.
When are you thinking of coming over? We are headed in to winter now - a lost of casual work will dry up and the chance of getting a full time job is slim.
Don't be put off, though - the diving here is spectacular and whilst you wouldn't necessarily earn that much it is a fabulous country to be in. You just need to make sure you shop around and get a position that suits.
Feel free to drop me a PM if you want some insider info on where and where not to work.