New Zealand is generally safe, with low rates of both violent and petty crimes. However, basic commonsense will serve you well, and you should always:
Lock your car! Crimes of opportunity are not uncommon, and locking your car and accommodation helps protect you and your belongings.
Keep your belongings on you! Unlike other places where pickpocketing or mugging are regular occurrences, these are unlikely to happen in NZ, and the safest place for your passport, wallet, and phone is in your bag or pockets.