Koalas only eat gum leaves
While it is true that koalas primarily eat gum leaves they have been known to munch on paperbarks like the Broad-leaved Paperbark and other non-eucalypts such as the Queensland Brush Box. They have also been observed eating bottle brush and wattle when flowering.
Interesting fact: Of the 600-700 gum tree varieties, only around 20 are preferred by koalas (including Blue gum, Gray gum, River Red Gum and Swamp gum) so they are finicky eaters. They like juicy leaves that are nitrogen rich and eat up to 500 grams a way on average. Little is known about the impact of drought on the nutritional value (or taste) of their food trees.