The leaf shape looks a bit like Persicaria strigosa or P. praetermissa but the flowers stems look as though they're glabrous, so I may be way off-beam!
It could also be Fallopia convolvulus (in the same family as Persicaria), which has leaves of a similar shape and flower stems not obviously hairy. In AVH there's a record of Fallopia convolvulus from the Gundaroo Public School collected in 1903!
The seed pods and leaves do look like a good match to Fallopia convolvulus https://upload.wikimedia.org/wikipedia/commons/0/03/Fallopia_convolvulus_RF.jpg
The more I look at the closeup of the flowers/fruit, the more inclined I am to call it Fallopia convolvulus because of the lack of obvious hairs on the 'stems' and no obvious branching of the inflorescence (cf the two Persicaria species).
Thanks! This ID makes a lot of sense, looking at other images available online it looks spot on. So this plant turns up in Gundaroo once every 100 years!?
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