I don't know about Australian English, but in at least one version of British English (as spoken by a British friend of mine), "bottom" has a more extensive meaning than it does in American English (as spoken by me): in AmEng it's a synonym for "rear end"; in my friend's BrEng it covers both one's "back bottom"="rear end" and one's "front bottom".
I don't know about Australian English, but in at least one version of British English (as spoken by a British friend of mine), "bottom" has a more extensive meaning than it does in American English (as spoken by me): in AmEng it's a synonym for "rear end"; in my friend's BrEng it covers both one's "back bottom"="rear end" and one's "front bottom".