All very interesting but doesn't answer the original question. I can only say what happened in Victoria, Murdo where the changeover occurred when they changed from an all numeral rego number to a combined alpha-numerical style. This was roughly 1953ish. Not only did the State change to alpha-numerical but they also changed the shape of the plates and the method of manufacture from a stove enamelled type which chipped easily to the current pressed aluminium rectangle.
In those days rego was handled by the police and in Victoria if you lived more than 20 miles from the Melbourne GPO your local policeman kept a supply of number plates on hand to register any local vehicle. I don't think there was a single change over day but slowly changed over a period as they supplied police stations with the new plates.
For the benefit of our English riders, the State provides number plates and you are supposed to hand them back if you let the registration lapse. You then get a fresh set of plates with a different number when you re-register your machine.
As far as I know each State still operated as an entirely independent entity. Co-operation between States was in it's very early days so what one State did had no relevance to any other.
Cheers,