Megafauna.....extinct!

Diprotodon
Weighing in at almost 2,700 kilograms, standing two meters tall and having a length of almost three meters, the Diprotodon was nearly 32 times as heavy as the Red Kangaroo; the largest marsupial alive today. It carried its young in a pouch, had wombat-like feet, and relatively long legs. It inhabited forests, woodlands, billabongs, and grassland where it grazed on all variety of vegetation. The giant marsupial Diprotodon was related to the kangaroo, but grew to the size of a present-day rhinoceros. The skull alone was over 1 m (3 ft) in length and was adapted for eating plants. The fossil remains of this giant marsupial are restricted in their distribution to Pleistocene deposits in Australia.
The Diprotodon was probably preyed upon by Marsupial Lions, Humans, and Megalania.

Megalania
Some people believe that the myth of dragons was inspired by the discovery of dinosaur bones. Others believe it was inspired by the Komodo Dragon; a three-meter-long lizard living on the Indonesian island of Komodo. Although the Komodo Dragon is quite large, it doesn't intimidate people anywhere near as much as the Megalania once did.
The Megalania was a lizard roughly the size of a Crocodile. Weighing in at almost approximately 940 kg and growing up to seven meters in length, it was able to tackle three-meter-tall Kangaroos, Wombats the size of cars and perhaps ate the odd human for a bit of variety in its diet. It was an ambush predator that probably waited near water for passing prey.
Like the Komodo Dragon , the Megalania was very efficient in maximising energy from its kills. While some mammalian predators might leave behind 25 - 30 per cent of its prey, the Magalania consumed almost everything, including fur, feathers, and ones. Not only would it have consumed almost everything, very little would have been excreted. Whereas a mammalian predator excretes between 32-37 per cent of what it eats, the Megalania only excreted between 8-13 of what it ate.

Marsupial Lion (Thylacoleo carnifex)
Pound for pound, the Marsupial Lion had the most powerful bite of any mammal that has ever lived. It was capable of inflicting a bite three times more powerful than placental lions twice its size. Estimates about the weight of the Marsupial Lion have varied. It was roughly similar in length and height to a Leopard, but it was more robust. Some estimates have put its weight at between 112 and 143 kilograms, which is similar to an average Tiger. The Marsupial Lions hunting style was probably similar to a leopard. They had strong forearms, and retracting claws that made it possible for them to climb trees. There they would wait for an animal to walk beneath them.