I can remember when Aussie military aircraft - DHC6 and B200 were registered VH-HP_ due to being leased from Hawker pacific!
Going further back than that (early '70s) I seem to recall that all the VH-H**, I** and J** marks were reserved for military use and not available for civil allocation in Australia.
From memory the Canberra bombers and Chinooks had H** identifiers, Caribous were JC* and Hercs were JL*. I can't recall what the Neptunes/Orions/DC3s used for 3 letter callsigns. I think at the time the Mirages/F4s/F111s were the only aircraft to have non-registration callsigns such as Buckshot, Falcon etc. Navy aircraft were always prefixed with "Navy", I think.
Anybody else remember what the allocation registrations were for military aircraft at the time?