Gall

Lots of things can cause galls on Eucalyptus in Australia. Various types of wasps are probably the most common - but other culprits can include thrips, psyllids, various scale insects, flies. From looking at the picture I would suspect that this is a wasp gall - but it is always better to see what comes out. Even that does not always give a clear picture, as no matter what originally causes the gall, there will always be small wasps in there as parasitoids of the gall maker.