Oh FARK man, i'm feelin really bad for you! All that hard work and waiting, especially when everything came out so good.
The mechanic is definately liable, as the car was kept in his care overnight, and should have insurance to cover such problems. Definately sounds suspect, it would be pretty dificult getting a bonnet over the fence, i would consider another mechanic once car is fixed again.
Why would you allow him to store your car outside?
At my mechanic it is stored inside, factory is alarmed and he has anti-ram post infront of the entrance. When he is tight for room he puts a car on the hoist and parks another one under it.
Look for signs of robbery. ie: Hole in fence, wires bent on top of fence, etc. See if you can determine wether it was an inside job.
Take legal action if he refuses responsability, first contact Victorian consumer affairs on 1300 558 181.
GOOD LUCK, and really hope it works out in your favour.
Alan