From what I can find out all Commonwealth vehicles up to 1990 carried a red "Z" prefix. This applied to every department from the PMG, Mapping, Mines, Health, CSRIO etc including Department of Supply. They normally had C of A in small letters on the plates. The military had their own number plates of just 6 numbers.
The Commonwealth now just register their cars under the State they are operating in or are delivered.
The "C" plates only applied to parliamentry and VIP vehicles eg the Prime Minister's car is C-1
Foreign embassies had DC 1234 for "Diplomatic Corps". They used to be immune from all traffic, parking and driving laws but they made such a welter of it (the Ambassadors kids were entitled to these plates as well) It was so bad that even if they killed someone it was almost impossible to charge them and they were on the next plane home. This has now been canned and they have to operate their cars like normal people.
The new military system has A for Army R for RAAF N for Navy followed by the year they were first registered then the multi digit individual numbers. EG an Army vehicle registered in 2024 would be A241234.
