Well it seems that we had been tricked, so the first thing I would like to do is apologise to the police.
It turns out that we had been lied to. We contacted the police to argue our case, and were told that it was the first time they had ever heard of such a thing. It turns out that this 'standard finders fee' as we were told was simply the garage trying to make some extra money from us! They obviously don't normally have people complain to the police about the matter!
Here in New Zealand, the police here are great when your car is recovered.
If they recover it and think there are forensic opportunities, then they will have it towed and checked over, then you do have to pay tow and storage costs.
That's what's happened here, although if I'm honest it would have been nicer if they would have asked us first. Apparently the car wasn't causing an obstruction, so they could have simply left it. It turns out it was just around the corner, so if we'd have known about the cost at the time, we could have gone and picked it up ourselves without even bothering the police in the first place.
Well anyway, the cost has now been halved thanks to this. So it's a good job we followed it up. It seems, therefore, that this thread has been started in complete error! Thanks for everyone's comments anyway!