Seeing a lot of detail in the comments about the CLOUD act which applies as they(fastmail) themselves have an equivalent that was signed between USgov and Australia.

The more concerning issue as far as Australian based tech is The Assistance and Access Act 2018 which

"...permits government enforcement agencies to force businesses to hand over user info and data even though it’s protected by cryptography.

If firms don’t have the power to intercept encrypted data for authorities, they will be forced to create tools to allow law enforcement or government to have access to their users’ data."

As far as i know this has not been challenged or walked back and with the rise of ChatControl like laws doesnt seem it will.

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.