In all honesty, how pushy are you being? You call once a day? I would consider the following, in this order:
1. The next time you go there, take a digital camera and take photos of your car, both where it is and how it looks. This is vital. As many photos as you can.
2. Increase the frequency of your phone calls and 'drop ins'. Make it inconvenient for them to continue holding your car. It's not unreasonable to be a little upset, assuming circumstances are as you've described, but be calm.
3. Document what you can, and then write the workshop a letter. Make sure you date it, then send it registered post. You can feel free to discuss the content of the letter with them once they've received it. You will need to find out exactly why there's a delay.
4. Get in contact with Graham Downie at the Canberra Times. He runs a consumer voice column and works wonders. I wouldn't bother telling the workshop that you've done this, let it be a happy surprise (you want them to wonder who else you are speaking to).
5. Call the ACT Dept of Fair Trading (http://www.fairtrading.act.gov.au) and relate your case. I don't know anyone there any more, but anyone there should be easy to talk to.
6. Hopefully it won't get to step 6.
They must be very happy having a hot car in the workshop all the time, must do wonders for potential customers. And to have been paid in advance! My God! I bet you won't do that again! The more annoying you are, the less time they will have to handle other customers. Be firm but polite: make it in their best interests to get your car off the lot as soon as possible.
And keep us up to date.