Wikipedia is very thin on details, this is all it's offering:
He was a cannibal giant, ultimately defeated by the resourcefulness of the family of a hunter he had killed.
Thardid Jimbo. (2017, July 2). In Wikipedia, The Free Encyclopedia. Retrieved 07:31, September 5, 2017, from https://en.wikipedia.org/w/index.php?title=Thardid_Jimbo&oldid=788611181
Why did the giant kill the hunter? How did the hunter's family defeat him?
TL;DR
He kills the hunter just because he hates and likes eating humans. The sisters defeat him by luring him into a cave and then burning the entrance.
A version of the story can be found on Myths and Legends of the Australian Aborigines, by W. Ramsay Smith. Unfortunately, the page isn't available in google books:
In the long long ago, in a cave in the land of Ge Rill Ghillie, there lived Thardid Jimbo, a great giant who stood ... (page 273)
Mummulberry, availing himself of this fact (shrug) came unperceived almost alongside the kangaroo he was tracking, speared it in a vital spot, killed it instantly, and dragged it away. When he had gone quite a distance from the other kangaroos he found a convenient spot and made a fire and cooked his prey.
In the meantime Thardid Jimbo, with rapid strides, covered a good distance, snatching up goannas and lizards that came in his way. Gazing intently upon the ground, he noticed the footprints of Mummulbery and the kangaroo. He followed the tracks, and saw the pole with the eaglehawk's wings tied to it. A little farther on he came to the place where the kangaroo had been killed. Still going onward, he no longer looked on the ground for footprints, because he could now smell burning wood and roasting kangaroo.
He continued on his way guided by the smell. Suddenly he came upon Mummulberry feeding on the liver and heart of the kangaroo. He stepped back a few paces and stood, watching him with big, wicked, and cruel eyes. Presently Mummulbery looked up and saw before him a great big being. He immediately recognized the being as Thardid Jimbo, the enemy of man, of whom he had heard so much. Mummelbery quickly took up his spear and placed it on the wommera ready to battle. Thardid Jimbo saw this action. He knew too well that even his brute force was no match for the skill and intelligence of man. Remembering that there were scores of spear-marks and many wounds upon his body caused by boomerangs during a pitched battle between himself and man, he sat down about ten yards away from Mummulbery. He made a fire, and enjoyed a meal of half-cooked goannas and lizards. Mummulbery placed his spear on the ground and sat upon a log, watching Thardid Jimbo, who was eating his food in a disgusting manner. Mummulbery, filled with pity, cut a leg from the cooked kangaroo and offered it to Thardid Jimbo, who quickly accepted it. he ate greedily, and crunched the bones between his powerful jaws, and swallowed both flesh and bone. Now it was at this time Thardid looked at Mummulbery's neck as if something had attracted his attention. He said, "I see a louse." Mummulbery, taken off his guard, asked him to remove it. Thardid Jimbo, seizing the oppurtunity, opened his mouth wide and bit Mummulbery in the neck, severing the head from the body.
Thardid Jimbo now threw the body of Mummulbery into the fire. After roasting it a while he took it from the fire and hung it on a tree. Then he thought he would follow Mummulbery's footprints to his house. The sister wives, hearing footsteps approaching, thought that their husband had returned. With faces smiling to greet him, they looked up and saw a sight that almost froze in their veins. For there, standing before him, was Thardid Jimbo, the enemy of mankind.
Thardid Jimbo said to the frightened girls, "I have killed a man yonder, and have roasted his body and hung it up on a tree." They realized that the victim was their husband, and tears began to roll down their cheeks. He said, "why do you look so sad? Why do you weep? I see tears in your eyes." "They are caused," said the wives, "by the smarting of the smoke." Thardid Jimbo said to them, "I have come to take you to my home and make you my wives." The elder sister said, "We have not tasted food, and we are faint with hunger. Grant us one favour, and then we will follow you. In yonder cave there lives a dog with his wife and family. We shall take it as a great favour to eat the flesh of puppies caught by your hands, O brave."
Thardid Jimbo turned and beckoned that they would follow him and see him bring the prize. When they arrived at the cave Thardid Jimbo entered it, and presently came out carrying the dead body of the (female dog), with the warm blood trickling from a wound inflicted by a blow of his mighty arm. He threw the corpse down at their feet, entered the cave a second time, and returned with nine puppies, a month old. He said, "Thy request is granted Fair One of the Dawn." Then the younger sister said pleadingly, "Will you be my brave and bring me the dog?" This so flattered Thardid Jimbo that he entered the cave a third time. The sister knew that the dog would not be easily captured, and that the pursuit would lead Thardid Jimbo to the far end of the cave, which was a very long one. This would give them an opportunity for action. After waiting for a few moments the younger sister called from the entrance of the cave, "O my brave, I am faint with hunger. Bring me the dog. I am craving for the flesh of the dog." She kept shouting and calling to him to bring the fog to her while the elder sister was busily engaged in closing up the entrance to the cave with boughs, sticks, and logs. The younger sister then crawled through these and helped the elder one to carry more sticks and logs. Then they both sat down to make a fire after the usual fashion, by rubbing two sticks together. When the fire was alight they placed it among the wood and branches, which caught fire and burned quickly. The smoke and heat entered the cave, and when Thardid Jimbo felt the smoke smarting his eyes he ran to the entrance. There he found a blazing fire in front of him. He made one desperate leap to get over it, but he fell short, and dropped into the fire and was quickly consumed.
