What is the most venomous plant in Australia?

It is called the gympie-gympie stinging tree. This piece of flora is regarded as one of the world's most venomous plants.

"Dendrocnide moroides," commonly known in Australia as the stinging tree, stinging bush, or gympie-gympie, is a plant in the nettle family Urticaceae found in rainforest areas of Malesia and Australia. It is notorious for its extremely painful and long-lasting sting. Indeed, it is sometimes referred to as the suicide plant because its sting is so misery-inducing that it has been said to cause suicides. The common name gympie-gympie comes from the language of the indigenous Gubbi Gubbi people of south-eastern Queensland.

https://en.wikipedia.org/wiki/Dendrocnide_moroides

As a Kiwi (New Zealander), I'll just add that this country has it's own resident tree nettle, known in Maori as ongaonga. I've only come across this plant once in many years of backcountry hiking and it really is a nasty piece of work with its own set of anecdotal history, such as killing horses and other stock.
https://en.wikipedia.org/wiki/Urtica_ferox