I was walking along the fire track this evening when I spotted this lovely big white mushroom cap, about 100 mm across.
Now, in colour and shape, this is not disimilar from an edible Agaricus. It is similar for example to Agaricus bitorquis. The location, in undisturbed eucalypt forest is a little unlikely for an Agaricus such as that however.
It can be seen that the edge of the cap has remnants of a torn veil. This might lead one to think that it is an Amanita. However, on flipping the mushroom over, all is revealed.
We can see from this that the mushroom has a ring on the stem, like an edible Agaricus and that the gills are within the right colour range, but on the surface of that ring is a deposit of orange spores. This shows us immediately that what we are looking at is not an Agaricus, but is in fact a Cortinarius of some kind. If it was an Agaricus, it would have a chocolate brown spore print.
Further differentiation from Agaricus is provided by the fact that the stem and the cap are part of the same structure. In Agaricus, the stem will break away cleanly from the cap.
As a rule, species in the genus Cortinarius are poisonous, so this is one mushroom that we would definitely avoid!