Amaranthus macrocarpus is a good fit, but not on list. Used description in VicFlora for comparison.
Decumbent long glabrous stems, leaves with an almost transparent mucro, fruits and seed as shown in photos. Not on ACT census list. Have sample for botanist to examine. A native plant.
A.macrocaprus isn’t on the list as it in an arid species - but I can add it. Assuming that is what Isobel finds it to be, I’d prefer it go to the herbarium before we confirm it here.
Arid zone but "Variety of habitats, mostly in depressions or low-lying situations." according to https://apps.lucidcentral.org/plants_se_nsw/text/entities/amaranthus_macrocarpus.htm and in VicFlora "Rather rare in Victoria, appearing only after summer rains along the Murray River floodplain downstream from about Echuca."