IM guessing I can't
 

Cash a US postal service money order in canada ?
and I correct ?

I don't see why not? I'm pretty sure I've cashed Canadian MO's here in the US.

(800) ASK-USPS

in australia you have to cash international money orders at the bank, usually with a feee, i assume it can be done there too

Im guessing no . because it says negotiable in US and possessions .

whatever possessions means .. I dunno

I dont think its international .

No you can't, they have two different kind of money orders, one for the US and one that is international.
I've had e-bay buyers send me those, even after warning them that they are not cashable outside the US.