Tua (37), who lost 26kg coming into the fight at a toned 107.8kg, showed brutal power to finish off Cameron sevens seconds into the second round after twice flooring him near the end of the first round.
The taller Cameron used his longer reach to land some left jabs early on, but Tua was soon landing some of his famed left hooks on Cameron, who offered little in the way of a guard to what has always been Tua's main weapon.
When Tua launched his first real offensive of rapid-fire blows with both hands, Cameron went down against the ropes.
Referee Bruce McTavish allowed him to continue after an eight-count, but as soon as Cameron was back on his feet Tua rained more punches to his red and swollen eyes.
Tua landed two blows while Cameron was reeling on his knees against the ropes. But McTavish overlooked those illegal punches and let the fight go on just long enough for Cameron to have some brief respite in his corner at the end of the round.
Tua pummelled Tua with 13 consecutive punches to Cameron's head at the start of the second round. Cameron was down again and what the promoters dubbed "New Zealand's Fight of the Century" was all over.
"I know I've lost a lot of weight and I think a lot of people have said I lost my speed. Now I believe I have just started my career, if anything," Tua said afterwards.
"So it was important for me to win this fight and win it well."
In recent years, Tua's career has been interrupted by a legal dispute with former manager Kevin Barry.
His previous bout was in September 2007, a second-round TKO of American Cerrone Fox, and anything but a convincing win tonight could have spelt an end to his boxing career.
The win takes his career record to 50 wins (43 by knockout), three losses (all on points) and a draw.
Tua walks away from Mystery Creek with a $500,000 cheque, and a possible crack at a world title next year.
Cameron was rated No 7 by the World Boxing Organisation (WBO) before the fight and lost his WBO Asia-Pacific and Oriental championship belts tonight.
The WBO's Asia-Pacific, Oriental and Africa zone chairman, Leon Panoncillo, had said Cameron's ranking and Tua's record made it feasible for the victor to go up against a world champion despite tonight's fight not being an official world championship eliminator.
