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.
These are bud galls on Black Wattle, which I believe are caused by Dasineura rubiformis. The species is described in Kolesik, P., Adair, R.J., and Eick, G (2005) "Nine new species of Dasineura (Diptera: Cecidomyiidae) from flowers of Australian Acacia (Mimosaceae)", Systematic Entomology 30:454-479. You can find photos on the web from south africa and portugal since this species has been released for biological control of black wattle. It was found not to affect the growth of plantation trees, but drastically reduces seed production and therefore invasiveness. Species is fairly common in Woodlands Historic Park.
4 images. Single specimen <5cm, growing in damp weeds in the car park. Cap diameter 2.5 cm. Cap disintegrating. Possible(??) same species as I observed in the area 4 weeks ago and posted on NatureShare. ID appreciated.