How do I get a "police clearance certificate"?

There is no such document in New Zealand. If you are convicted of a crime, a Criminal Record is created. This is the New Zealand equivalent of a ‘police clearance certificate’, ‘police record’, ‘police file’, ‘police check’, ‘personal record’ or ‘criminal check’. If you are asked for any of these, a copy of your Criminal Record  from the Ministry of Justice should meet the requirements. If you have no convictions, you will receive a letter stating that is the case.