Translation:Excuse me, where is the bathroom?
Because this can be an urgent need under certain circumstances, it may be helpful to know that another commonly used word for toilet/restroom/bathroom/washroom/W.C. is 厕所 (cèsuǒ).
What does “washroom” mean to you? “Bathroom” is a very fitting translation for most American dialects of English at least, where it, like Chinese 洗手间 is a euphemism for the room where the toilet is.
If you forget the Chinese word for bathroom you probably have more luck asking where the toilet is in English. Almost all bathroom signs in China is labeled either with Wash Closet or Toilet so most Chinese probably know it by English.
The problem asking for the bathroom in Australia (in a private house) is that you may get pointed in the direction of the bathroom not the toilet. They are often separate here. (Although if you're a Yank, we'll assume you want the toilet.)