I was helping an Australian friend troubleshoot this issue, and it seems like this only affects connections from Optus to Square Enix servers in Japan, such as for the game's launcher/login screen, the Lodestone, and forums.

Doing a traceroute to the lodestone for instance clearly showed the traffic dropping after going from Optus through the Level3 network to Tokyo, whereas connection tests to the NA and EU datacenter worked without a hitch.

Unfortunately it seems that Final Fantasy XIV's login process requires passing through the unreachable Japan-based servers first before you connect to the datacenter servers elsewhere. We worked around that for now by switching to a phone hotspot or VPN to get past the launcher login, then switching back to regular internet before connecting to the datacenter to choose a character. Doing it like that retained the login session from the launcher and prevented a disconnect from the datacenter servers due to changing IP's/routes.

So it mainly seems like Optus needs to figure out why traffic routed to the Japan-based Square Enix servers is not working properly to get this fixed properly...