Indeed, people have drama'd a lot about Australia's surveillance laws, but they have little applicability to Fastmail. Additionally, if you are worried about government surveillance, your safest bet is just to be storing your data under someone else's government.
If you live in the US, it's very easy for the US to get your data, and more irritating for the US to ask Australia for your data.
If you live in the US, it's very easy for the US to get your data, and more irritating for the US to ask Australia for your data.