The Super Rugby Pacific club was placed into liquidation on Monday after their current owners announced that they would not continue to fund the franchise beyond 2026 on April 15.
Unless a last-minute saviour comes in to salvage the club, only introduced in 2022, they will fold and the competition will shrink to 10 teams in 2027.
But Pasifika (2-12) relished in the emotionally charged occasion at GIO Stadium on Saturday and snapped a 12-game losing streak to avoid a record-equalling 13th loss.
The cellar dwellers' victory wasn't smooth sailing, as the Brumbies led both on the scoreboard and numerically when barnstorming inside centre Faletoi Peni was handed a second yellow card and sent off in the 63rd minute.
But the hosts couldn't make the most of their opportunity and substitute Melani Matavao stormed over in the 73rd minute to claim victory.
"With all that had happened during the week, in terms of Moana going into liquidation, we (knew we wouldn't) see a lot of our group that we left behind," Umaga said.
"We talked a lot about our strong purpose of who we represent. We're representing our culture, our people, and we saw that come through in the end.
"(In all) the emotion of the last few months, today, I couldn't tell you what happened in those other games ... I can tell you what happened today, and I'm very proud of it. This one stands out."
The Brumbies (7-7) needed a big bonus-point win to help them move as high as fourth on the ladder, but instead will have to settle for sixth.
Their faltering lineout and a series of errors will be a concern for coach Stephen Larkham as the Brumbies now face the table-topping Hurricanes across the Tasman in the qualifying finals, where they must win to secure a semis berth.
"There's a whole lot of frustration in the change room at the moment, there's lots of guys thinking about what they could have done differently or better," Larkham said.
"For me right now it (my message is) get some good sleep, that's the priority, fully recover from this game, and get your details right when we come back in."
The Brumbies came out firing, with Rory Scott planting the ball over the line before Tom Wright crossed in the 16th minute.
But when veteran James Slipper was handed a yellow card, the visitors capitalised through Patrick Pellegrini before the flyhalf wrestled over for his second to tie the halftime score at 14-all.
The Brumbies re-established the lead when they shifted left and found Luke Reimer in the corner before Peni was given his marching orders after a high tackle on Andy Muirhead.
ACT's newly re-signed Klayton Thorn thought he had scored, but a forward pass in the lead-up denied him a try, and Matavao got on the receiving end of some slick passing to steal victory.
