I'm sure that Australian Federal Police and Victoria law enforcement would be able to exercise search warrants on Fastmail's servers if they needed to.

Since you're in Tennessee (and thus a U.S. Person), you're actually ineligible for collection under FAA 702. Gmail/Hotmail/Yahoo, etc. would actually be the safest place for your information.

Of course that assumes that you believe the NSA follows U.S. law. If you don't have that assumption, that also implies that the NSA wouldn't respect Australian sovereignty enough to keep it from infiltrating Fastmail's servers.

I'm saying this to illustrate that there's no silver bullet for secure transmission and storage of information.