SA - Yurebilla is billed as 3 days, not exactly rural though. KI has a 2 day coastal. THe Heysen trail is somewhat serpiginous in the Fleurieu Peninsula and the Southern Flinders, so one can plan loops using that as a base. Eg, Mt Remarkable national park (Heysen crosses it) has several possibilities from Mowbray Creek to Melrose; or around Wilpena. The Adelaide Hills has some longish walks which can be linked together. See
http://www.southaustraliantrails.com/tr ... pe=walking NSW has lots of gems; the only through walks i've done have been supported but that's not a bad idea because the hassle of getting back to the starting point is removed. Recommend LifesanAdventure for the Six-foot track (Katoomba to Jenolan, fabulous) although the lodge
http://www.6fttracklodge.com/ can assist - I think you leave your vehicle at Katoomba, they take you to Jenolan, overnight at the ecolodge and arrive back at your car. Lifesanadventure also do the RoyalNationalPark which I'd absolutely recommend, although as it starts at Bundeena (ferry from Cronulla) and ends near another railway station it can be done unassisted. the fantastic WildWalks site has a lot of suggestions, and many incorporating bits of the GreatNorthWalk (Syd=>newcastle) have a trainstation near each end.
Going soon? In Tas, Maria Island or Bay of Fires (less interesting IMHO) might be easier in winter than the plateau options, can go solo or supported; both have a number of providers. The Tasman Coastal track /3 cape walk has formal drive-in campgrounds as well as the walker only.
WRT Qld - yes, the Thorsborne sounds wonderful; you may find the Gold Coast Hinterland one (GreenMountains to Springbrook via BinnaBurra) on the lamington plateau a bit more accessible. I've done days 1 and 3, need to organise a through walk..... I LOVE the GreatWalk behind MacKay. Its supposed to be 5 days but it isn't, even for a slow coach like me; i've done it as a series of day walks & half the fime managed the "day" walk return (ie x 2) with hours of daylight left.
Enjoy your trip