I'd have thought the closest thing to a tortoise would be one of the available turtles. Both are super cool incidentally. I've played with tortoises in the USA and Asia, and am currently looking into the viability of setting up a tortoise ranch in Asia. Walking distance from where I am now there's a tortoise which free ranges every day and comes home every evening. I found it wandering around in a local village, figured it was an escaped pet (an African tortoise in South East Asia which had no fear of me and was very friendly... seemed a safe bet), so after watching it for a while and taking some pictures of it wandering around and eating weeds, I tried to find the owners, who I found and to my amazement said they let him out every day like a cat, he explores and wanders around, and in the evening comes home. Equally astonishing to me is that by local standards he's worth about two month's wages, and no one has stolen him!
Anyway... (as you can tell, I'm also pretty fond of tortoises! I could rave on about more tortoise stories...), Shingle-backeds are pretty much just a Blue-tongued Lizard, except you want to give them a hot (proper hot) basking lamp and you want to keep them very dry, like, touch the ground and it's hot and dry type thing. If they are in high humidity (which is normal humidity by normal reptile standards) they get respiratory issues. Keeping them outside is hit and miss, and I've seen many more sickly ones which died outside and ones which stayed healthy long term. In Toowoomba you might get away with keeping them outside if you design the enclosure very carefully. The temperatures are great for them where you are, but finding a dry microclimate will be tricky. Keeping them dry enough during wet spells in Toowoomba will be your challenge. Any other type of Tiliqua will be an absolute piece of cake in Toowoomba, inside or out, and that's what I'd be inclined to go for. Or one of the available turtles - super cool, and much more similar to a tortoise anyway.