G'day Kev, and welcome to the forum. We have a few of your countrymen that lurk around here from time to time.
In GENERAL, the .308 Winchester is a good cartridge for 300 meters (and beyond) and for which it is relatively easy to find ammo/ components. However, you may also want to look at some of the 6mm family of rounds, as well as things like the .204 Ruger.
Some of the factors guiding your choices will be availability where YOU live, price, and just what you want to do with it. While I enjoy shooting some of my older MILSURP rifles, for 300 yard bullseye matches, I would not be pulling out the 91/30s or the SMLE's. For casual plinking? Oh HELL yeah!