Given the reported size it is almost certainly Phlebopus marginatus. The underside, rather than having gills, would be spongy with numerous tiny holes, putting it into the group of fungi known colloquially as boletes. There are various nondescript, brownish boletes, but locally there is no other bolete with that some chunky appearance and of that size. Fully mature specimens range in size from less than 10 cm in diameter to around a metre. The small specimens need a microscopic analysis to be sure they are this species, rather than some other nondescript, brown bolete. A native species, the original description of which was published in 1845, based on a specimen collected in Western Australia.
I have seen quite a few in the Sanctuary at Tidbinbilla, but did not know the species, so thanks for the Id. I will upload my sightings along with a number of other fungi found at the same time.