Mackenzie last took to the competitive stage in 2010 in the 100kg-plus open class but had become frustrated with being the only bodybuilder competing in that weight range.
"It's great to be in the Masters and have somebody up there to compete with now."
The next two Wellington regional championships could also see Mackenzie qualify for national finals for the NZIFBB in Auckland on September 29 and NABBA/WFF also in Auckland on October 6.
He said after last weekend's victory he had spoken to wife Rachel, a past title-holding bodybuilder in her own right, and would extend his contest regime to include the NABBA nationals next month. "I can't get bigger but I can get better - lower my body fat and improve my condition. I want to be proud to be 40 and I'm in the best shape of my life," he said.