Both sides defended defiantly in a match for the purists at Hamilton's Waikato Stadium and the contest remained try-less until the big All Blacks lock barged over the line two minutes from time.
Chiefs flyhalf Damian McKenzie, who is expected to return to the All Blacks squad when it is announced on Sunday, had few opportunities to display his running flair but kicked five kicks from five attempts to keep the home side in front throughout.
The Chiefs topped the regular season standings to earn the right to host Saturday's final against 13-times Super Rugby champion Crusaders, who crushed the Auckland Blues 52-15 in Friday's first semi-final in Christchurch.
"I feel like we've won every way this season and that was just a hell of a grind against a good Brumbies team," said Brad Weber, who next Saturday will lead the Chiefs in their quest for a first title since they won back-to-back in 2012-2013.
"I'm stoked it's against the Crusaders. If we want to be the best, we've got to beat the best. Been chasing it for 10 years and I just want it so bad."
The Canberra-based Brumbies, looking to become the first Australian team in nearly three decades of Super Rugby to win a playoff in New Zealand, had just two penalties from the boot of Noah Lolesio to show for their considerable efforts.
The players warmed up in teeming rain that returned intermittently throughout the 80 minutes and the match was dominated by long kicking as the teams jostled for field position in the greasy conditions.
McKenzie scored the first points in the eighth minute with his first penalty and the flyhalf had the cow bells clanging again when he doubled the lead with his second on the half-hour mark.
Lolesio got the Brumbies on the board from the kicking tee a few minutes after coming on as a replacement for flyhalf Jack Debreczeni and the Australians held out repeated Chiefs attacks on their try line in the last few minutes of the half.
The attrition continued after the break and McKenzie and Lolesio traded penalties with the former then pushing the Chiefs 12-6 ahead with a huge 50-metre effort in the 72nd minute.
McKenzie eventually got a chance to run free in the last five minutes but was just unable to put fullback Shaun Stevenson away for a try.
It was left to Rettalick to finally find a way through the Brumbies defensive line a few phases later.
"We certainly threw everything we had at it," said Brumbies skipper Nic White.
"Fair play to them, they're a good side."