On my shelf at the moment:
Almost Human by A A W Wilkie, subtitled "Reminiscences from the Melbourne Zoo". Oddly no date of publication, however from notations it must have been prior to 1932. I just opened it to find three feature-size newspaper articles from around 1980 and a brochure which must have been prior 1960.
Zoo Walkabout by Vincent Serventy, pub 1979 covering mostly major collections.
Royal Zoological Society of South Australia 1878-1978 by C E Rix pub 1978 a history.
The Zoo by Peter Crowcroft pub 1978. Crowcroft was director at both Taronga and Brookfield in the USA, so this book covers more than just Australia. However it was published in Australia so it is very heavy in Aussie, and especially Taronga, content.
Zoos of Australia by Catherine de Courtney pub 1994. Not very thick, published by Australia Post to coincide with a series of stamps featuring Aussie zoos.