As far as I am aware the age is 8 years Oz wide. Melbourne defenitly has 8 y/o cabs. I rode in one recently!
It is different in NSW and other states where a Taxi must be less than five years old. Here nobody is going to buy a used car to turn into a Taxi.
The problem with ex govt cars in recent times instead of selling them at under 40k or 18 months they are now doing 60k plus. Look at a Pickles auction list. Actually bad business as less new cars [the ones that were Australian made] are sold and less late model cars go back into the economy. What they used to sell for meant that government was actually getting the cars for free, or almost free as they do not pay Sales Tax or stamp duty.Now they are tearing up on resale and have to maintain them too. Though that is often far too little. Some of these 60k cars have ever only had ONE service in 2-3 years. And when sold are sometimes running bald tyres, worn out brakes and other faults. Though in the past the worse cars were the Police cars at under 40k. And I am sure they are worse now.