As far as I can tell, it changes nothing, assuming that your email was sent as plain text. Although Fastmail stores your email on encrypted disks [1], it of course has the encryption keys for these disks. Even before the new law, they would have been subject to any Australian search warrant requiring that they hand over your email, and would not have had any technical reason for not complying.
In general, unless you and your correspondents are using PGP or some such, your email is readable by anyone who can obtain a search warrant in the country where your email provider resides. (Protonmail may or may not be an exception [2]).
In general, unless you and your correspondents are using PGP or some such, your email is readable by anyone who can obtain a search warrant in the country where your email provider resides. (Protonmail may or may not be an exception [2]).
[1] https://www.fastmail.com/help/ourservice/security.html
[2] https://www.wired.com/2015/10/mr-robot-uses-protonmail-still...