I will start working in China on a Z visa from January.

So you plan to visit HK when your Z is being changed into resident permit? If so, I am afraid that you can't do this. When you change your Z into resident permit, your passport is already handed over to the PSB. Without your passport, how could you leave for HK and get back?

I've got dual nationality and have a German passport as well. The z type is in the British passport. And what I need to do is a trio from Hong Kong to Australia and/or Germany. I heard Hong Kong was the answer to my problems, but can't quite work it out. If there's another solution I'm all ears.

Uwe, if you entered with your British passport, then it's impossible for you to use your German passport to exit. The immigration officers will check the entry stamp. However, you didn't have any entry stamp on your German passport because you didn't use it to enter.

You can ask them to stamp both your passports. They might not do it, but it doesn't hurt to ask.

I believe there is a temporary travel document available but it is for emergencies which you do not qualify.
I very much doubt your German passport would be 'recognized'.
Your best chance would be to request the soonest possible processing with frequent follow-up by your sponsor.