John - I have done extensive research and there is no evidence whatsoever, of any of the Australian military Austin Champs, being used by the RAAF, anywhere, at any time.
Possibly the major reason is that the Austin "Truck, 1/4 ton, 4x4, CT", as it is referred to in both British and Australian military terminology, was designed as a Combat vehicle - thus the "CT" (Combat Truck) designation.
The official Austin designation was FV1801A (Cargo) and FV1802A (Utility body).
There were 5 (or possibly 6) further military variants on these models, plus the Austin A90-engined civilian version, which was designated the WN3.
Only about 500 of the civilian WN3's were built and nearly all were exported to Australia.
The WN3's can be distinguished by the fixed windscreen, as compared to the folding windscreen of the military versions.
The Austin "Champ" name was originally only applied to the civilian version, but the "Champ" name became the generalised name from sometime around the late 1950's.
WarWheels.Net- FV1801A Austin Mark 1 "Champ" 1/4 Ton Truck
I can find no reference anywhere in news media of the Australian Govt military vehicle purchase deal involving the initial 400 military Austin Champs - despite articles describing other major military equipment purchases in the 1950's.
It is also mentioned on numerous websites and forum discussions that another 400 used military Champs were purchased in the late 1950's for the Australian Army.
The initial 400 Austin FV1801's purchased by the Australian military were modified from the British models to meet Australian Army specifications.
These changes involved some body strengthening, a guard for the brake master cylinder, and altered radio mountings.
In the newspaper link below, is the only (poor) picture that I can find of an Australian Army Austin Champ. The AWM has no information whatsoever on the Australian Army Champs.
The original designation of "Truck, 1/4 ton, 4x4, CT" was changed to "Truck, 1/4 ton, 4x4, GS" in the Australian military in 1958.
The Australian Army Champs were regarded as unreliable, expensive to maintain, and thirsty. Their reputation wasn't helped by any repair work on the RR engine necessitating the removal of the complete engine, and its return to the RR service centre in Sydney.
15 Nov 1960 - NEW ARMY UNIFORM AND EQUIPMENT UNDER STRENGENT F...