Yet another Australian expression of surprise, disbelief ar anguish. There is quite a collection of these (see tags below).
Usually said with each word pronounced very separately and deliberately, but is quite often uncontrolled. The first thing I said when I turned on the TV and saw the World Trade Centre collapse was:
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"