Unknown fungus

Description:

With caps up to 50 mm across, this mushroom had a thin brown cuticle with fine scales. Caps had a prominent umbo which were still visible even in mature caps which developed a concavity. Gills were crowded with dark spores. Stipe was smooth with some spore staining near the cap. A fine membranous annulus was seen half way up the stipe. Younger caps (pic 4) appeared to have a pink tinge with very thin cuticle.

Habitat:

Spotted in mown open grass at a nature reserve. Eucalyptus trees close by.

Notes:

Looking for an ID.