OK, I went to Team Capital Holden in Wellington but as the motor was missing, a new unit (you can't get the motor without the whole mirror unit) is NZ$370. Anyway, they had to order the part so I would have to wait until tomorrow while the part was ordered from their parts centre. I decided to go back down to where the car was hit and sure enough, on the other side of the road was the mirror and motor unit, undamaged but wet. I quickly turned back and went to see what they could do. The mechanic at Holden had just finished his shift but said he would stay on to try and salvage the mirror unit. Ten minutes later he had fitted it and made sure it worked okay and they charged me a grand total of $0. Yes, zero charge. How's that for service!
In summary in case anyone wants to know, NZ$370 + GST + labour is the actual cost of replacing a mirror if you smash the glass and motor unit.