There are two different plants in the photo. The ones with the large leaves are probably a Malva; the upright reddish-green plants look like a species of Amaranthus.
See https://canberra.naturemapr.org/Sightings/4282415 showing the red stems of Amaranthus. Both plants had low horizontal stems and were tangled together.
The new photos indicate how large the plants are - taller than the stumps to keep cars out!also there are lots of plants stretching along the road into Yarralumla.