G'day Paul.
Re Kangaroo Island... Terrific little Station type C.P. at Jervis Bay, top of the Hill just before the Ferry (cant think of the name) or beaut C.P. at Rapid Bay a little further back. Over on the Island, a good spot to prop is on the far Western end. There is lots to see using this area as a Base....N.P. camping at Rocky River, in the Flinders Chase N.P. but we found it's full of backpackers and is run by over the top Greenies who dislike Caravans and will site you on a bitumen road giving all the good spots to tent people. My preference next time will be the privately owned Western KI caravan Park & Wildlife reserve just outside the N.P.
Another great spot is at American River. Dont know of any free camping areas on the Island..Quite a few self registration spots but not free.
Kangaroo Island is well worth at least a weeks stay...2 weeks would be better particularly if you like fishing.
Jock