The real reason why we are not allowed to keep them is down to 2 things.
1/ the risk that exotic species become established environmental pests and
2/ the risk that they carry diseases that have yet to be discovered or have yet to be properly tested for and managed
I am not terribly concerned about point 2, but as for point 1, I would definitely be concerned. We have numerous pests, including some carnivorous ones, all of which have wreaked large scale ecological consequences regardless of their biology or where they originated from.
Feral corn snakes, to give you one example, would probably decimate bird and reptile populations, and maybe small mammal populations too - particularly the smaller species, and corn snakes are also well known to attempt to eat other snakes too (most reports are cases of cannibalism but it still proves the point), so that too is also an inherent potential threat.
I wouldn't trust corns in Australia, especially in the northern NSW and QLD seaboard areas where they would obviously do very well - this of course ultimately boils down to an almost perfect temperate-to-subtropical climate that corn snakes would adapt to without any trouble at all because that is the climate they are most commonly found in.
In saying this, I couldn't see a problem with snow cornsnakes (these are basically a white snake with pink eyes and with a very light pink/orange/yellow pattern on them). Other white corn snakes would fall under that category too. I can't see how these would pose any considerable threat. Normal coloured corn snakes, that is a different matter altogether.