I haven't read all the replies and our situation is slightly different because we're not in NZ

But we have live in, unqualified domestic help here in Singapore. The things I was worried about before we hired M were - lack of privacy (for us and her), communication issue as english would be her 2nd language (not such an issue for you if you're getting a German au pair), different expectations and the huge responsibility on OUR part - we are required to pay a bond to the government here in case M runs away or gets pregnant
It's worked out GREAT so far, I'm so glad we did it! We have a babysitter available so DH and I can have time to ourselves, help with housework and cooking, help getting C to bed when DH works late .... the list goes on!
The privacy thing hasn't really been an issue as we have a two storey house and our rooms are upstairs and M's room is downstairs with her own bathroom. I wrote EVERYTHING down in the beginning to avoid confusion/misunderstandings and tried to be really clear about the things I knew would get on my nerves .. ie I would ask for help if I needed it with the kids (I'm a SAHM), break times, days off, rules around having guests here etc etc.
It's not perfect of course, I have to repeat myself fairly frequently and things I think are obvious and common sense are clearly NOT obvious to M sometimes

But she works hard and is great with the kids when I need her to be so I try not to complain too much!