Australia surely has its fair share of introduced weeds and invasive plants. On this trip I have already seen a few that I had only heard of but not seen before, and others that are much more abundant here than further south.
Opuntia is one of the textbook cases of an economically promising plant turned into horrible invasive weed but also of biological control done well. I remember even as a teenager seeing a documentary that showed first near-monocultures of Opuntia cacti covering the Australian landscape and then the devastation the biological control agent - a moth - caused in the populations of the cactus after it was released. There are memorials in this area celebrating the defeat of the cactus and the efforts of the moth. Wikipedia has a nice little entry on the issue.
Above the flower of a low-growing specimen. Apparently numerous species of the genus had been introduced, so I am not sure which one this is.
One of the plants I had read about but not seen before is Hypochaeris microcephala (Asteraceae), one of several weedy species of its genus introduced to Australia, but the only white-flowered one.
Finally, a Bryophyllum (Crassulaceae). We saw it frequently along roadsides, but most of the plants were past flowering and presented merely shrivelled, blackish stems. I assume this species was introduced deliberately as an ornamental.