In an earlier post, I traced the imagery of the "Boxing Kangaroo" as a symbol of Australia's fighting spirit reports of an Australian-versus-American boxing match in 1891 (The Kangaroo Whops the Eagle), through the worldwide kangaroo boxing craze that began with "Kangaroo Jack" in Melbourne, Australia in 1891, and soon found its way to London (1892-93), and eventually to the United States (1893), via Hawaii and London.
But since the "boxing" instinct in kangaroos is a natural phenomenon from time immemorial, the first kangaroos were probably associated with "boxing" as soon as humans, familiar with the sport of boxing, encountered kangaroos.
Humans familiar with the sport of boxing were unaware of kangaroos until British Explorer, Captain James Cook, "discovered" the existence of kangaroos and laid claim to Australia for England, in 1770.
Quite possibly the first published description and image of a bare-knuckled "boxing" kangaroo appeared a few decades later, in 1806:
There are at present (1806) a remarkably fine pair of kangaroos in the exhibition rooms at Exeter ‘Change. They were brought over from Port Jackson in New South Wales, and have been in Mr. Pidcock’s possession between six and seven years. The male, when in an erect posture, is upwards of six feet high, and is an animal of prodigious strength. On visiting the menagerie some months since, I saw this noble quadruped wrestle with the keeper for the space of ten or fifteen minutes, during whith time he eveince the utmost intrepidity and sagacity; turngin in every direction to face his opponent, carefully watching an opportunity to close with him, and occasionally grasping him with his fore paws, while the right hind leg was employed in kicking him upon the thigh and hip, with equal force and rapidity. The struggle was indeed obstinate, and the keeper acknowledged that the animal was sometimes almost superior in point of strength. When the contest was at an end, the kangaroo still continued to present himself as ready for a fresh engagement; nor did he seem willing to return to his apartment, till the female was brought out to entice him. He then returned, bounding through the exhibition room with astonishing speed and vivacity.
Rev. Thomas Smith, The Naturalist's Cabinet, or Interesting Sketches of Animal History, Volume 1, London, James Cundee, 1806, pages 235-236.