Well IIRC they have found fossil platypus relatives in argentina, so you are probably at least half right, if not totally, they (or at least, early monotremata) were definitely present on gondwana, prior to the geological splitting of the supercontinent, presumably whilst the arctic was not so cold as it is now (platypus are endotherms, but with a much higher tolerance for cold, and for longer periods than are reptiles)
Its kind of ironic really, about australian life, they have the nastiest of anything, bar spiders (the most venomous, or certainly most dangerous being a far larger relative of the recluse spiders, the genus Sicarius, a similar, powerfully cytotoxic venom based on sphingomyelinase-D, not highly aggressive, at least from reports of those in the exotic pet trade, but lightening quick, and if you DO get bitten, there is absolutely bugger all that can be done, the venom is similar in action to that of the viperine snakes, rotting flesh and causing massive systemic clotting of the blood, rarely ever come into contact with people so bites are rare, not australian natives, african with one exception, found in south america, but beats the shit out of any ozzy spider for sheer unpleasantness)
But for all the funnelwebs, mouse spiders, taipan, tiger snake, brown snake etc. and giant centipedes, the place evolves scorpions to completely fuck the usual order of things where scorpions are concerned. They tend to take two overall 'design patterns', either with delicate, thin claws and correspondingly, with less physical strength and durability, evolve much more potent venom (E.g deathstalkers and many other Buthids, as well as Hemiscorpius, (H.lepturus in particular), which unusually for a non-Buthid scorpion, has extremely potent venom, and again unusual, its a cytotoxic, necrotizing type venom, almost all scorpions relying on neurotoxic cocktails in their stings), the other option being scorpions with massive claws, built to smash and crush, and physically rip their prey apart, like the big hefty imperial scorpion, venom no worse than a bee sting reportedly, but with claws like boxing gloves, so they don't really need potent venom.
Ozzy scorpions seem to have a mix of both, but all, but all of them seem to have pretty weak venom, even the australian Buthidae (almost all clinically dangerous scorpions are Buthids, Hemiscorpius being the main exception, with H.lepturus of iran, causing over 80% of fatalities due to scorpion sting, which is quite something considering the other main dangerous scorpion common to the area being Androctonus crassicauda, one of the deadly fat-tailed scorpions [note-Parabuthus spp. are also known as fat-tailed scorpions, but are only related due to being both members of the Buthidae family, and unlike Androctonus, Parabuthus can fire venom from the sting, like the arachnid equivalent of spitting cobras, although perhaps more akin to the mangshan viper, which doesn't direct the venom with the great accuracy of cobras, more flinging or flicking it towards the target]
