Has a mini tsunami hit Mandurah?
Check this storm surge graph
www.transport.wa.gov.au/imarine/mandurah-tide.asp
I'd post the image but right underneath it says that's illegal.

From midnight the storm surge varied by 20cm on an hourly basis, (nine times from midnight to 0900). That's a lot of water moving around very quickly, can that just be caused by barometric pressure changes and changing wind strengths. Or is some clown playing with the float?

edit.
OK looks like it's just the wind, it follows a very similar pattern and the barometric pressure looked fairly steady.