Hey Mark, the reason it’s separate is that Christmas Island (and Norfolk) is counted as a ‘country’ on iNat. I agree it would be nice/convenient if all Australian external territories were lumped into Australia, but in the meantime it’s pretty easy to string them all together in one search. Here’s a journal post I wrote about this a while back: https://www.inaturalist.org/journal/thebeachcomber/64297-setting-up-projects-with-australia-as-a-place-filter
the ones that are currently separate are:
Norfolk Island
Christmas Island
Cocos Islands
Heard Island and McDonald Islands
this explore link gets you everything (Australia + those 4 territories + also Australian waters): https://www.inaturalist.org/observations?place_id=6744,7333,7616,10287,10293,118147