Another Aussie - as with the others, usually toilet, loo, dunny etc.
At work, I'll refer patients to the toilet, bathroom or "facilities".
When socialising, I may say "I'm off to the little girls room" or possibly "I'm going to scare some little boys" (but I think that one may be a bit of an in joke - if the ladies room has a line-up, I'm not beyond using the men's at a pinch - always warning them first: "female incoming, close your eyes!" Yes, I know...
When all's said and done, we're a pretty casual mob.
I always ask my dog if she needs to "have a leak" on a trip. She knows when I say that, she has to make at least a show of doing so before we continue. (Oh, and she actually cocks her leg like a boy. Like mother, like daughter!) *snerk*