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Crocosmia montbretia
Showy orange/red summer flowers
Stream bank
Kate is right. It is Crocosmia that is considered an invasive species. It grows wild in grass lands and on roadside here in Victoria. But, it is an attractive plant.
I'll put in the ID. We request that you edit your spotting to put in the ID and information as to where you found it under "habitat". Information on spottings always helps with an accurate ID and is useful for students and others who use this project . Thanks.
I would say that it is most likely in the family Iridaceae, but I don't know enough about the Aussie flora to confirm that it is a Crocosmia. I did find that there are some Crocosmias in Western Australia:
http://florabase.dpaw.wa.gov.au/browse/p...
This looks like it might be Crocosmia aurea (an African species) - was this planted in a garden?