car insurance

To be safe and cheaper Get a Third party Insurance that will cost you roughly around $ 30 per month ( depends on your car )

I agree thrid party is cheaper but it only covers cost on damages to other peoples vehicles and property. If you damage your car then you are liable for all costs needed to repair your car. If you're new to NZ then I would suggest a full cover, you can get two kinds
1. Agreed value which is where you pay a flat rate on a certain value
2. Market value is where the market value of your car goes down over time and the cost of your insurance goes down. Personally it all depends on how much you pay for your car. If it's a cheapie then 3rd party is probably best but if its 5k upwards then you'll need to think about the kind of insurance you're going to need. Also age has a bearing. If you're under 25 then your insurance will be more expensive as that age bracket is a large cause of accidents on NZ roads. Another thing is how much excess you are going to pay if you have an accident. This is a fee you must pay before any repair work is carried out on your car.
The two biggest insurers are State and AMI insurance.

is insurance a must? can i drive without having one?

You can drive without insurance, but if you have an accident and it's your fault and you damage other peoples property/car, you'll be paying for two lots of damages. Repairing bodies of cars in NZ is very expensive and you'll probably be paying 2 or 3 times what your car is worth. So consider that 4-500 bucks will save you thousands if you do have an accident. It's a gamble one way or the other, personally I think 500  a year for total piece of mind, is money well spent.