As you mention, 5th Wheelers for LV's are OK in all States an Territories on C or LR license and have been for decades except ACT.
Up until about 30 years ago the most common configuration was horse floats but once it became viable to 'convert' the Yank or Canadian models to ADR compliance (240 Volt/LH door/LPG/etc) the Aussie demand exploded and several Aussie Companies now make some very nice units.
Demand in Aus is so high now that the Yanks et al make ADR compliant models ex factory and Aussie models are made by Jayco Fifth Wheeler, Southern Cross Fifth Wheel Caravans, Winjana Fifth Wheelers, Travelhome, Venture Fifth Wheelers and Dryden Trailerhomes.
Back to the good old ACT...
The ACT had a different wording to the other states in their Licensing Act so it was indeed illegal to drive them into the ACT unless the Driver had the appropriate HC License.
The rigs themselves were always quite OK
Obviously this was affecting Tourism etc so this anomaly was rectified a few years ago now
http://www.legislation.act.gov.au/es...f/db_50929.pdf
Bottom line is you can now go anywhere in Aus with a fifth wheeler on a C or LR license (as determined by the tow vehicle)