Olive/Abyssinian thrush?

Interesting! From a bit of checking, Olive Thrush (and as far as I can see, all the other similar species) should have a distinct dark patch on the bill around the nostril, which Abyssinian lacks, so the latter would be my bet - a quick Google image search suggests this is present even in juveniles, so should be noticeable on any fully-grown bird.

Yes I read that too, but I've since seen photos labelled as olive thrushes which lack the dark upper bill. They may, of course, be mislabelled though - particularly as the taxa have been grouped together in the past.

I'm going to assume this was in the Australian bit? Is this what you thought was an American Robin?

@Brum Yeah it was sitting hidden under the table in the zebra finch aviary of the Australian walkthrough. We were about to walk away when it was pointed out to us, and we were told it was an American robin.

I'd go Abyssinian per HBW's notes, but sounds like separation is still debatable. I can see why it could be called an American Robin, but it certainly is not.