Wow, I do wonder how frequent this would be.
A few months ago the Melbourne Cacti society had a representative from the dpi give a talk on succulent weeds. There's so many potential weeds, potential illegal plants. They don't focus on plants that are out of control, blackberries, lantana, etc. they focus on plants that are almost unheard of but do have great potential to become invasive. So there's loads of illegal weed plants most of us have never heard of.
I doubt the Big W would have been aware of the plant, but their supplier should know about this sort of thing.
I was in Goulburn a few months ago where they have a terrible problem with serrated tussock. I was working on an organic farm, one of my jobs was to walk around the paddocks with a mattock, digging up any tussocks i could find. It was everywhere, i felt my efforts were completely futile. I'd hate to see another evil plant like this around.