If the launcher says Americas, it is easy to assume that means US ping. A player discussion on Blizzard’s forums shows why that distinction matters when planning a group across Australia, New Zealand and North America.
Will Oceanic players use Australian data centres?
Blizzard’s developer Q&A at 1:12:35 says Oceanic players will mostly connect to Australian data centres. Forever’s realmless setup does not mean that all players in the Americas region share one physical server location.
What happens when I play with US friends?
The same answer says Oceanic players can play with friends in the US and share the auction house for their ruleset. When a group spans different geographical locations, it may connect somewhere else, so latency can vary. Grouping still requires the same ruleset and faction.
That answer does not give a guaranteed ping or say that the group leader always determines the data centre. If your regular group crosses the Pacific, try your usual party together and see how it feels before making promises about latency.
Does this confirm Australian routing for the stress test?
No. The Q&A describes the broader setup; it does not specifically confirm how the stress test will route Oceanic players. The official stress-test notice gives regional start times, not a physical data-centre location.
For now, separate the two questions: Blizzard has described Australian data-centre support for Oceanic play. Exact stress-test routing and the latency of your particular cross-Pacific group remain unconfirmed.