It is worth noting that most mechanics are a member of an association or chamber of commerce, due to the professional and financial benefits that can be gained. e.g. VACC, MTA etc.
Although these associations are there for their members (the mechanics) they need to maintain their integrity and also protect the good mechanics from the bad. Therefore, they take any complaint against any of their members very seriously and investigate it thoroughly. They want to ensure that you, as a customer are happy, and that any bad mechanics are weeded out of their system.
When I last had a bad experience with a mechanic, I called the VACC (VIC) and was pleasantly surprised by how seriously and professionally they handled the matter. The mechanic who had done the wrong thing by me, and who refused to do anything about it, very quickly righted the situation as soon as the VACC was involved. It was amazing the difference one phone call made. I was also very impressed by how detailed the VACC got, and how they followed up.
A list of the associations, for the various states, can be found here:
http://www.motor.net.au/