Annual or perennial forb to 45cm tall. Leaves usually alternating up the stems, 0.5-2cm long, 1-5mm wide, flat, more or less hairless, with more or less rounded tips. Flowers small (less than 3mm across), tubular, with pink to purple lobes, in groups of 3-7 at the bases of the leaves.
Differs from Lythrum hyssopifolia in that the flowers are are in groups of 3-7.
PlantNET description with line drawing: http://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Lythrum~wilsonii