In a cheeky piece John Birmingham recently defended “our precious freedom… our inalienable right to drop pants.. butt cheeks clenched in angry defiance.” Lest anyone be misled, bending over and baring your buttocks to the passing parade of citizens in a public place is almost certainly illegal in Queensland.
You could be charged with public nuisance (section 6 Summary Offences Act) or an indecent act (section 227 Criminal Code). If children are watching it might be exposing a child to an indecent act (section 210 Criminal Code) Lean over far enough to expose your genitals and that’s another offence (section 9 Summary Offences Act).
Whether baring your buttocks or anus is disorderly, offensive or indecent may depend upon all the circumstances.
What is disorderly? “Disorderly” means considerably disturbing or considerably annoying to others. It may even mean something less than “considerably” but that at least will be enough; Mbuzi v Police  QDC 257 at .
What is offensive? The Chief Justice of Australia recently explained, “The ordinary meaning of the word ‘offend’ encompasses vexing, annoying, displeasing, angering, exciting resentment or disgust. Its various registers are defined in terms of the reaction of one person to the conduct of another rather than by that conduct itself… In the law, however, the need to determine whether expressive conduct is offensive… is not necessarily met by asking the victim. They may take offence… too easily. So the law tends to engage the services of that leading light in the judge’s small band of imaginary friends — the reasonable person. Other members of that band include the ‘right-thinking person’, the ordinary prudent man of business, the officious bystander, the reasonable juror properly directed and the fair minded and informed observer.”
To be offensive the behaviour must be ‘calculated to wound the feelings, arouse anger, resentment, disgust or outrage in the mind of a reasonable man’ who is ‘reasonably tolerant and understanding, and reasonably contemporary in his reactions.’; Ball v McIntyre (1966) 9 FLR 237 at 243-245.
What is indecent? That all depends on the context. An indecent assault must have some sexual connotation; R v BAS  QCA 97. Indecent language need not. But it is not the job of the law to punish lapses of good taste or good manners; R v McBride  QCA 412. Ultimately, it is a question for a jury to decide whether something offends against community standards of decency; R v Jones  QCA 19;that is, the standards of ordinary decent minded but not unduly sensitive members of the Australian community; Phillips v Police (1994) 75 A Crim R 480.