Cooper landed a right hook high on Dunnett's head to send his opponent crashing to the canvas inside the opening round of his scheduled four-round cruiserweight bout at the Brisbane Entertainment Centre.
Dunnett, 32, had two previous professional boxing bouts in 2010 but it was in Muay Thai boxing, in which fighters are allowed to use their elbows and knees.
He looked well short of fitness against Cooper, who will be
calling the shots for the Reds in this season's Super Rugby provincial series, which kicks off next weekend.