Hey there and welcome to the forum.
Funny you asking for the australian cop cars. I recently found out it is a registry switch that does enable the aussie liveries.
It is the mysterious "Variant" switch.
Veg, in his wisdom, also ported this variable to his modern patch (without knowing what it actually does).
So if you using the modern patch you only need to set "Variant=4" in the nfs4.ini - seems this works with every game version too.
But prepare for disappointment - this will trigger the AUS.TGA to be used on some USA tracks only; it will only work ingame, not in the main menu. Cars without AUS.TGA will keep their default CAR00.TGA livery.
This actually is how the aussie version of High Stakes works...
On a full game installation the registry switch is at this path:
HKEY_LOCAL_MACHINE\Software\Electronic Arts\Need For Speed High Stakes\1.0
Change DWORD-key "Variant" to value "4".
So thats the way for a regular game installation or when using the Stock-/Expansionpack.
Greetings