The 37-17 win sends the Chiefs to a final against the Crusaders next weekend in Christchurch - their third final appearance in a row.
McKenzie kicked 22 points from six penalties and two conversions, his only miss - ironically - from one of his easier attempts of the evening.
He even found time to make one of the best defensive plays of the season, stopping a seemingly certain Tom Wright try, when he slid in and got his arm under the ball to deny the diving Brumbies fullback.
By that stage, with 15 minutes left, the game was more or less over, thanks to the shots at goal and a pair of tries to Emoni Narawa.
The winger, who will hope for an All Black recall, found himself with plenty of work to do around the ruck. His first try came off some smart play that collapsed the Brumbies defensive screen, then repeated the same sneaky run off the resulting kickoff to put the Chiefs back on attack straight away.
It wasn't one-way traffic though. The Brumbies opened the scoring through prolific hooker Billy Pollard, who dove over for his 11th try of the season. Not to be outdone, winger Corey Toole picked up his 12th, with a great finish off a crosskick.
The Chiefs held a 19-12 lead into the break, but that was cut, thanks to an even better finish by Toole, who smashed his way over McKenzie for a finish that belied his 80kg frame.
That was as good as it got for the Brumbies, who watched McKenzie knock over another two more penalties in the next five minutes, then Josh Jacomb slide over for the crucial last try of the game.
The old adage of a 'game of two halves' could be applied to this one, as Chiefs once again committed to openfield kicking early and often, before the game broke open in the second, as they chanced their arm a bit more.
We'll likely see a repeat of the safety-first gameplans that have dominated the playoff series so far in next weekend's final, with the Crusaders having no qualms about putting boot to ball either.
This is a disappointing end to the season for the Brumbies, who would've felt very confident, after their impressive win over the Hurricanes last weekend. They lost starting first-five Noah Lolesio, after a failed HIA in the first half, and while replacement Jack Debreczeni certainly didn't do anything wrong, they clearly could've done without that disruption.
Some questions should be asked around the circumstances of the headknock Lolesio took, though, with the TMO opting not to take any sort of close look and the TV coverage failing to screen a replay.
