Sandgroper is just a shibboleth for Perth & WA people, I think, but it might also be generational. I'm old enough that Fat Cat & Humphrey B Bear mean something to me and are a shared experience, but probably not for people younger than me.

Helpful off-topic instructional video: https://www.youtube.com/watch?v=QWv3L6J1YVE

But saying sorry as a pre-emptive "oops didn't mean to step in front of you, mate" (when the other person had clearly walked into you), that has existed in Perth as long as I can remember.

How about Humphrey B. Flaubert (or Humphrey B. Flow Bear, if you prefer)?