But their ninth straight win against Australian opponents said more about the young Brumbies and their mission than it did about anything to do with the Super Rugby's uncertain future.

The new-look ACT roster is building its foundation on guts and determination after withstanding a Waratahs onslaught in the first half before Henry Speight blew NSW away after half-time.

It was the sweetest of wins, claiming the inaugural Dan Vickerman Cup and lifting the Brumbies to back to back triumphs in Sydney for the first time since the start of Super Rugby in 1996.

There has been speculation about the Brumbies' future in Super Rugby as SANZAAR officials weigh up whether to cut one Australian and two South African teams from the competition.

The Brumbies have been dragged into the off-field saga and there have been rumours of a potential merger between the ACT side and the Melbourne Rebels.

It's hard to believe the Brumbies have even been mentioned in the axing conversation, but their win against the Waratahs was a message of intent for their 2017 championship goal.

"Coming up to Sydney to play the Waratahs is probably our hardest derby game and it's a very satisfying win," said Brumbies coach Stephen Larkham.

"In terms of the [SANZAAR] decision, it's irrelevant. We're celebrating a great victory against the Waratahs so that's what we're focused on.

"I think the decision [on cutting teams] has already been made. The only thing that's important tonight is the way the boys played and the fact we put another good performance out there."

Winger Speight proved the difference, igniting the game with two bits of individual brilliance in the second half.

The first was chip-and-chase to break the game open when the scores were deadlocked at 7-7 and the second came in the final 10 minutes when he busted through some lazy defenders to seal a 16-point win.

Speight's superb attacking play was the diamond in the Brumbies' performance, but their defiant defence set up the result.

They were reduced to 14 men when Scott Fardy was sent to the sin bin early in the first half and the Waratahs took advantage to open up an seven-point lead.

The match was punctuated by handling errors and miscued attacking moves in Sydney's wet conditions, but the Brumbies' lineout and scrum rose to the challenge to inflict pain on the Waratahs in classic Australian derby.

Fardy was immense in his comeback after being rested last weekend and the Brumbies' win sets up a massive clash against the Otago Highlanders in Canberra next weekend.

"We knew coming up here was always going to be a battle, but it was a good effort from both teams," said Brumbies captain Sam Carter.

"Henry is in unbelievable form at the moment. When he gets going he's very hard to stop."

The Waratahs suffered a major blow when flanker Jack Dempsey suffered a suspected broken leg and is likely to be ruled out for up to 12 weeks.

"I'm bitterly disappointed with that, I thought we were the better team in the first half. Sustained pressure killed us," said Waratahs skipper Michael Hooper.

"Teams like the Brumbies will grind you down and that's what they did. It's back to the drawing board [for the set piece].

"The turnaround from South Africa wasn't a factor ... we couldn't get it in the second and that wasn't due to fatigue at all."

AT A GLANCE

ACT BRUMBIES 28 (Henry Speight 2, Tevita Kuridrani, Robbie Abel tries; Wharenui Hawera 4 conversions) bt NSW WARATAHS 12 (Tom Robertson, Jake Gordon tries; Reece Robinson conversion) at Allianz Stadium on Saturday night. Referee Glen Jackson.