Gall caused by a rust fungus, about 10cm across, on a large Acacia Mearnsii.
Update: Uromycladium tepperianum has been split into at least 16 different species each infecting a different range of Acacia species. The one on Acacia mearnsii is called Uromycladium murphyi. See "Diversity of gall-forming rusts (Uromycladium, Pucciniales) on Acacia in Australia" by C. Doungsa-ard et al, Persoonia vol. 40, 2018 pages 221–238.