See 'Mud and Dust' pages 167 and 168 for an explanation of the integration and role of philanthropic organisations supporting the AMF in SVN.
Army registrations were issued for administrative reasons such as the issue of POL and stores, movement through military channels to and from South Vietnam, and so on. The vehicles were supplied and still owned by their respective organisations.
By the time of East Timor, the policy had changed, and such organisations had to register and maintain (at least in theory), their own vehicles. The Salvoes Land Rovers in East Timor carried Queensland state registration, for example. They were still moved to and from theatre with military assistance.
Mike C