All a rounders are difficult. Anything that works well on big game tends to be messy on small game. The 357 lever action previously mentioned is a decent possibility, a little light on the big end, a little heavy for the small end, but capable if kept conservative.
Much depends on what type of deer you may hunt. In the US, we think of our deer when answering this question, but the deer of Australia are much different. What deer are in your area?
Red deer are almost the size of our elk, and I would not use a 357 for hunting them, were as fallow deer are a lot like our whitetails, and the 357 can work at conservative ranges and with the right bullets.
I prefer lever actions over bolt guns, But I don't find a single cartridge I like when thinking of all Australian hunting possibilities. I would get a pistol caliber lever action, and cut my hunting teeth on smaller game, then down the road I would pick up a heavier lever action for big game if ranges are short, like 150m or less, or a bolt gun if ranges are much longer.
For a red deer and a lever action, just take your pick of 405 winchester, 444 marlin, 45-70, or 450 marlin.
If I were to go to AUS to hunt everything with leverguns, I would take a Rossi carbine in 45 colt, and my marlin guide gun 45-70. Cowbaoy action level loads for small rabbit type stuff, full power and +p loads for the middle ground, then switch to 45-70 for deer and ending with 45-70+p loads for buffalo.
