I was driving on the SH16 the other day and didn't notice the transition from 100kmph to 80kmph on the approach to Auckland. As I turned round a bend, going about 90kmph, there was a traffic cop with a handheld speed gun. Not entirely sure it was pointed at my car, as there are 4 lanes, but does anyone know how the handheld speed guns work?

Do they take photos and issue infringement notices later or does the police officer have to pull you over in order for you to be fined? Also, are demerit points issued for these demeanours?