A 30-30 lever action is simple, quick, and will get the job done inside 200 meters. I favor the Marlin design. I mention the 30-30 because it's not expencive, but there are quite a few other cals in Lever Action design that will do the job and are a lot of fun. Ammo availability might be the deciding factor for you. See what your local shops carry.
I've seen Land Owner carry a Winchester mod 70 fetherweight in 270, that I drooled over, that cal will take any game you have in Oz. but wouldn't be my choice for your buffalo there, though it would work. The advantage of the 270, 30-06 is distance. A dingo running off with a baby at 250 meters would be a job for a bolt action in the 270, 30-06, 300 win mag, If you want to shoot that dingo coming into the camp for the baby, a Lever Action in 30-30, 35 Rem, .357, 44 mag, 45 Colt, 45-70, 38-55, would be a better choice.
All of the rifles mentioned by others above will do just fine.
Let us know what you decide on. Sorry to hear about all the gun law changes that have happened since I left Oz. It's a shame.
Enjoy your hunts.
Fish