Can confirm, I'm living in Sydney with the home internet on Optus NBN, and Japanese FFXIV servers are completely unplayable while American FFXIV servers are perfectly fine. I'm getting around the problem by using mobile data.

I don't know anyone else with Optus playing FFXIV, but friends who are with other ISPs have no such problems.

When contacting Optus about it, their first reply was an inappropriately enthusiastic message about their Game Path service. Leaves a sour taste when considering that this entire thing may be a deliberate attempt at pulling new Game Path subscriptions.

May be shopping for a new ISP soon.