Yes, Bathurst is an open road most of the year. The speed limit is 60kmh all the way, and the road being what it is gets more than its fair share of police patrols etc. Across the top, even 60 will open your eyes in a standard road car.
The track day at Bathurst is $1590 to enter in regularity, and $2590 for super sprint. Both are 2 day events, but still several times more than the usual track day cost at a regular circuit. Many treat it as a bucket list item.
http://www.challengebathurst.com/
The NSW Hillclimb Championship usually holds 1 round at Bathurst each year, and I assume would cost less. However, being a hillclimb, your track time would also be considerably less, and not the whole circuit. This year they ran "The Esses" and "Mountain Straight", and combined, would still be less than half a lap.
As for other circuits, I think Phillip Island is probably the best of the mainstream circuits. A lot of car clubs run track days here, as well as National and State level races, not to mention Moto GP and Superbikes. Costs to enter a track day at PI will be much lower than Bathurst, depending on which event you attend with. For example, an MSCA track day will cost $225 this year, but you must also be a member of one of the clubs that form MSCA ($85 in my case), and you need a CAMS L2S licence (about $110). I could not tell you whether a licence from a UK body has reciprocal rights or not.
http://www.msca.net.au/index.php?opt...d=1&Itemid=102
What I'm not sure about is whether you are looking to run a "regular" hire car (!), or run with an organisation that arranges track days with their own fleet - obviously more expensive ...