At various times of year, including mid summer, the parks around Perth erupt in large amounts of mushrooms. These grow in circles that can be 10 or more metres in diameter.
Agaricus arvenses forming a large ring in a park in Perth.
Viewed up close, the mushrooms have a distinctive appearance.
These mushrooms have a smell of almonds that varies in intensity, depending on the location. Some smell so strongly of almonds that they can only be used as a flavouring. The almond smell is due to the presence of benzaldehyde. This has been shown by gas chromatography.
These mushrooms are white gilled initially and then the gills turn to dark brown with age. They also bruise yellow, which is often taken as a sign of inedibility. In fact, however, the yellow staining mushrooms that must be avoided are Agaricus xanthodermus and other species which contain phenol. To be able to eat these mushrooms with confidence, one needs to be able to distinguish between the smell of phenol and the smell of benzaldehyde. That is the smell of phenyl disinfectant and the smell of almond essence.
I find that the small mushrooms are the best to eat. I have seen other people collecting these. Some elderly Italian gentlemen. I have also grown this mushroom. Well, just one small one!
Here is a picture of a small one, taken in Dagleish, Perth, on 29 March 2010.
In fact, these were the first mushrooms to be put into cultivation, before the normal Agaricus bisporus, and if the early attempts at cultivation had turned out differently, we might be used to the taste of almond mushrooms.
Since these mushrooms grow in the open sunlight, it is interesting to speculate whether they contain significant quantities of vitamin D, since the development of this vitamin has been demonstrated in other members of the genus when exposed to ultraviolet light, as described in this slide presentation.
Here is a few that I picked one lunchtime that are sitting on my keyboard in my former office in Perth.
Agaricus xanthodermus, the one that has the phenolic smell, is not something that I have encountered often. I did come across this bunch of them growing in sand at Yeagarup though on 5 May 2007. The smell was very distinctive as was the yellow colour of the base of the stem. 
December 2023
It is interesting to look back at this post now. Agaricus xanthodermus, or more correctly Agaricus section Xanthodermatei have pretty well taken over SW Western Australia now. I see them everywhere, in parks, on farms, in gardens and roadside verges.
One last comment. These mushrooms only have white gills at the immature stage. One should be very careful to ensure that the gills turn brown with age. Never eat a mushroom with white gills in the belief that it is a field mushroom unless you have established that you are looking at the immature stage of an Agaricus. That will take some experience. Failure to heed this advise could be fatal!
Mature specimens of Agaricus never have white gills. The deadly Amanitas do.