Posted to Flickr as Asclepias fruticosa, it seems like these plants were misidentified in the park where Vicki took the photographs in two different ways. The first was a relatively minor error. I recalled from this plant identification thread on the UBC BG Forums earlier this year that the convention today is for African milkweeds to be placed in the genus Gomphocarpus, while American milkweeds (American in the sense of the New World) remain in the well-known genus Asclepias. Examining photographs of Gomphocarpus fruticosus, though, pointed out a mismatch – the fruits of Gomphocarpus fruticosus are tapered. Fortunately, Weeds Australia (and the similar plant on the UBC BG Forums thread above) suggested what I think is the proper identification, Gomphocarpus physocarpus, or, commonly, balloon cottonbush, bladderbush, wild cotton or paina-de-sada. Of course, as often needs to be said with common names, these names are misleading. This species does not produce the cotton of textile use, but is so named because of the cottony appearance of the pappus attached to the seeds.
Native to Kenya, Tanzania, Mozambique, South Africa and Swaziland, Gomphocarpus physocarpus has naturalized elsewhere in the tropics, to the point where it is considered invasive in Hawaii, Norfolk Island, and New Caledonia (source: Pacific Island Ecosystems at Risk). Both the Western Australian Flora and the New South Wales Flora Online provide detailed factsheets about this species: Gomphocarpus physocarpus via FloraBase and Gomphocarpus physocarpus via PlantNET.