Very interesting! I'd been led to believe that koalas were non-existent outside of Australia until London and a few others imported them in the eighties and nineties. Do you know where in the zoo the 1880 animals were kept?
It was kept somewhere offshow in the Clore, but I'm not sure where. I know that until recently the Whipsnade squirrel monkeys were kept in an offshow cage just about visible from the right angle, behind the gentle lemurs. Perhaps this is where they were kept before?
I fully agree that it would be great to see tree kangaroos return. They would be a good use of the empty Tecton Roundhouse, in my opinion.
EDIT: just searched the gallery and found
this photo by
@Crowthorne which seems to suggest that, while onshow, they were free-roaming in the walkthrough portion of 'Rainforest Life.' If true that's quite brilliant as I don't know of many tree kangaroo walkthroughs, but it does make me disappointed that I have no memory of them there whatsoever. The only photo I could find of her in Rainforest Life is
this one, supposedly taken the day she went onshow.