Having the same issue.

I just did a ping test on the 2 JP data centre servers and ran them for a minute each.

neolobby05.ffxiv.com (124.150.157.156) had 61 sent 53 received at a 13% package loss

neolobby01.ffxiv.com (124.150.157.158) had 62 sent 49 received at a 21% package loss

Did a typical one with google as well even using the US site despite my DNS probably resolving it back to AU. Had 0% packet loss after a minute.

The fact that it only happens during prime times I'm guessing means Optus's route to Japan is getting conjested as using a VPN to connect to Japan still gives me the same issue. Maybe they just don't have the desire or funds to have an as efficient route / connection to JP.