Always seems like a great idea, but your pets will actually be much safer if they live in some sort of enclosure or pen. As Snowy says, domestic animals are a threat, a visitor can always leave a gate open etc etc etc...
There is also the problem with sleepy lizards that they come from west of the Great Divide, where the conditions are very much drier. On the east coast, respiratory problems with these lizards are common, and may be exacerbated by being left out in the humid Brisbane summer environment - the disease may become very advanced before you even notice it if you have less contact with them. (Notwithstanding the extremely dry start to summer so far, when it does become wet & humid, it may kill your lizards).
Keeping them indoors with artificial heating ensures reduced humidity, and less potential for disease to take hold.
Jamie