His game plan was clearly to cautiously work his way into the fight, but Betham, who lost by decision to Shane Cameron in his last fight, was far too classy.
Spencer said: "It was as tough as I thought it was going to be, probably tougher. He showed his true class tonight.
"It took a long time to think about this challenge because Monty is a good mate and great boxer. It was a one-off."
In another undercard fight to the Joseph Parker main event, Chiefs prop and crowd favourite Ben Tameifuna lost in a controversial split decision against new Manly league player Willie Mason.
The Claudelands Arena crowd reacted with surprise at the decision, with two judges awarding the three-round super-heavyweight fight to Mason, and one giving it to Tameifuna. Many believed the first round was even, but that Tameifuna had easily done enough to win the next two and therefore the fight.
Even Mason expressed his shock, saying: "I don't think I won that, I think Ben beat me."
He added: "Sorry Hamilton."
Switching between orthodox stance and southpaw, Tameifuna, who had tipped the scales at 142kg compared to Mason's 118kg, regularly hit his opponent with powerful right hands. He conserved his energy superbly, and even had the professional's knack of appearing to score points at the end of each round.
Tameifuna, who raised his hands in triumph after the final bell, and wore his Mooloo-coloured mouthguard over his right ear as he waited for the result, thanked the crowd for coming and apologised for the result.
"I wanted to improve on last year, that was my goal."