The large Australian lizard that disappeared around 50,000 years ago was Megalania, formally known as Varanus priscus. It was a giant monitor lizard and the largest terrestrial lizard known to science.
Megalania lived during the Pleistocene, when Australia supported an extraordinary community of giant animals, including Diprotodon and other megafauna. Fossil remains are fragmentary, so estimates vary substantially: many specimens may have measured around 2 metres excluding the tail, while larger reconstructions have proposed total lengths of 4.5 metres or more.
Its closest living comparison is probably the Komodo dragon, though Megalania was much larger. Modern research shows that some monitor lizards possess venom glands, so Megalania may have been venomous, but that conclusion remains inferential because the animal is extinct. It was neither a dinosaur nor a mosasaur, and its disappearance roughly coincided with major Australian megafaunal losses and the arrival of humans.