Jane Have a look at https://apps.lucidcentral.org/plants_se_nsw/text/entities/silene_nocturna.htm - it has bifid petals, but is said to only flower at night
I checked the link to Lucid: and the Silene with bifid petals called Silene nocturna certainly looks like my photograph, which was taken in early evening, about 6.30 pm. Silene nocturna is not on the ACT Census.
Here is another link which suggests S. nocturna has been previously seen in the ACT, but not recently http://keyserver.lucidcentral.org/weeds/data/media/Html/silene_nocturna.htm
A single herbarium record in the ACT, according to ALA. Collected on 27 October 1968, from "40 m uphill from the end of Strickland Crescent" (which sounds like Red Hill.)