Sorry, I didn't mean to. I think the definition of Australasia is not clear in my mind at least. This threw me a bit.You lost me. You imply a link between this and PB's comment about long-tailed koel?
Faunal regions and their distinctness may have their roots in the movements of tectonic plates - or may not.
Australasia | Realm & Subrealms | One Earth is one definition and I am sure there are many others.
The fact that the southern part of New Guinea and Australia share a tectonic plate may also be a reason they are seen as one region by e.g. RAMSAR.org, but I don't know.
They list Oceania as Australia, Fiji, Kiribati, Marshall Is, New Zealand, Palau, PNG, Samoa and Vanuatu. (See Countries | The Convention on Wetlands, The Convention on Wetlands. This may be because they only having Ramsar sites in those countries.)
They don't include many south Pacific islands or any north of the equator, or Indonesian Islands in their definition, so Australasia and Oceania are clearly two different entities, each with more than one definition.
Too much confusion for my brain to cope with..
https://www.ramsar.org/country-profile/vanuatu