So you had the Retrofit done?
The reason:
In 1989, the Australian Government restricted the use of refrigerant R12 under the Ozone Protection Act, thus requiring replacement of the use of R12 with the environmentally friendly R134a gas.
R12 to R134a conversion (retro-fitting) for older vehicles conditioned to use the R12 gas, as R12 gas is no longer available to consumers.
Think there is more to it than just changing from R12 to R134a.
Also any shop that is still doing R12 would be putting their livelihood at risk
Edit: Had mine retrofitted about 16months back. Going strong and cold