Someone posted that or similar on another thread. A comment up thread mentioned that O2 MVNOs usually allow EU roaming for free, the others may charge. But sounds like some of the EE ones might be following suit.
There’s about 30 MVNOs in Oz(!). Here’s an article (albeit a year old now).
Untangling Australia’s web of cheaper mobile providers and MVNOs on the Telstra, Optus and Vodafone networks.
www.reviews.org
They don’t discuss international roaming, so the recommendation are presumably purely domestic ops. One advantage of the Telstra based telcos is the potential for wifi calling.
As far as I know, none of the Oz MVNOs will provide full cellular support for Apple Watch (or smartwatch via eSIM using a common number). Which doesn’t affect me for now.