All depends on which species you are referring to when you say tree monitors. We have a fair few species of monitor here in Australia which you could classify as 'tree monitors' (broadly speaking due to their more arboreal habits), with most to the exception of the canopy goanna (Varanus keithhornei) being kept in captivity at varying levels of availability.
Probably the two most common and easiest to obtain 'tree monitors' in Australia would be the black headed monitor (Varanus tristis tristis) and the pygmy mulga monitor (Varanus gilleni). Stripe-tailed monitor (Varanus caudolineatus), Bush's monitor (Varanus bushi), freckled monitor (Varanus tristis orietalis) and spotted tree monitors (Varanus scalaris) are all available and kept, but all are a bit more difficult to come across and may require you a bit of investigation to find someone who is breeding them or has an adult they're willing to sell. I was fortunate recently to have a friend of mine breed his spotted tree monitors so I was able to pick up a hatchling from him. As mentioned above emerald monitors (Varanus prasinus) are kept in Australia, however they are currently restricted to only a handful of collections and are not readily available yet. The people who have them are aiming to breed them up and release to the public, it may just be a while until they do so.
Hopefully this helps you.
Cheers, Cameron