.22. .223's have a lot of fans, as does 243 and 308 as mentioned. 30/30 is pretty popular too. In the Europeans 6.5x55 is common. Less common though still numerous is the 22/250 and 30/06.

Fav brands are winchester, sako, ruger, Brno, remington.

Lot of ex mil fans out there too. Years ago the 303/25 was all the rage, and while plenty of rifles still exist, you cant buy factory ammo for it.

Check out usedguns.com.au (I think) as it will give a good idea and picture of what is generally available.