Not sure what the rules are in Austrailia for buying firearms or what ammo is easy to find and some rounds are chearper than others in different markets.
Any of the WWI or WWII Military rounds with a soft point bullet would be a good choice. or any of the up or down calibers of those rounds.
So to start with the WWI / II rounds, 303Brit, 8X57, 7X57, 6.5X55, 6.5 Carcano, 6.5 Japanese, 7.7X58 Japanese and 30-06.
Desendants of the WW ctgs 308, 243,358 Win, 270, 35 When, 6X57, 6.5X57
Then you have all the European, English, and American hunting rounds like 7mm Mag, 300 H&H, 300 Win Mag, 35 Remington, 338 Fed, 338 Win, and even the tried and true 30-30.
All of these ctgs will carry enough energy to kill a pig from 150 pounds to 300 out to 300 yards if you are able ot shoot that far. The cheaper the ammo in hunting bullets the more you will be able ot practice and the more confidant you will be at longer ranges.
I know that some will offer 223 or other small bore, small case option. And they work. I just prefer a heavy for caliber bullet.
My personal favorite Deer and g caliber is 308Win with 180 grain soft points where legal and 150 grain lead free where I have to use lead free projectiles to save the Stupid condors