The Assistance and Access Act is completely irrelevant to Fastmail, because Fastmail doesn’t offer end-to-end encryption. Fastmail was always subject to the Telecommunications Act, which allows Australian police access with warrants, and Fastmail has always made it clear that it complies with legal warrants.

The article you're quoting [1] concerns itself with the creation of systemic "encryption-breaking" capabilities and exploits which said law bends over backwards to expressly prohibit [2].

[1] https://fee.org/articles/australia-s-unprecedented-encryptio...

[2] https://classic.austlii.edu.au/au/legis/cth/consol_act/ta199...