63D outline the requirement:

A provider of an age-restricted social media platform must take reasonable steps to prevent age-restricted users having accounts with the age-restricted social media platform.

There isn’t really a definition of what reasonable steps are but that looks like it’s on the commissioner to define them under section 27 of the act. A service “includes a website” (does that include gaming servers?) so Lemmy sites would most certainly fit the bill. It really depends how onerous these reasonable steps are as to whether Lemmy sites will be able to implement them.

Not sure how the act implementers plan to deal with servers hosted in other countries. Will they block them? If not then this is a paper tiger, but also an impediment to further development of Australia based platforms.