The Brumbies have sent a powerful message to the ARU with a watershed 28-12 Super Rugby triumph over the NSW Waratahs in Sydney.

The emphatic win not only secured the Brumbies the Dan Vickerman Cup but also saw them register a ninth straight derby success in the very week in which Australia's most successful Super franchise's future in the competition was called into question.

But coach Stephen Larkham, having already hit out at talk of the Brumbies merging with the Melbourne Rebels as SANZAAR considers axing an Australian team in 2018, refused to fuel the fire after Saturday night's win at Allianz Stadium.

Believing a decision on the Brumbies' future had already been made - one way or the other - Larkham said his opinion on the turmoil engulfing Australian rugby was "irrelevant".

"Today we're celebrating a great victory in Sydney against the Waratahs. All those decisions are not ours to make so we'll focus on our win here," he said.

"I don't really want to answer questions about that tonight. It was a fantastic game of rugby out there and I think that's what we should be talking about."

Two tries in two minutes midway through the second half, including a scintillating effort from winger Henry Speight, blew an otherwise dreary match wide open as the Brumbies recorded back-to-back away wins over the Waratahs for the first time.

But it was a forgettable night for the Waratahs, especially flanker Jack Dempsey, whose season is feared over after he was taken from the field on a medicab and in a leg splint after landing awkwardly in a heavy tackle early in the second half.

The Tahs' forward stocks were further stretched when Test prop Sekope Kepu, in his 100th match for NSW, failed a concussion test after copping a head knock.

The score was locked at 7-7 before the Tahs lost Dempsey and Kepu.Two minutes later, the Waratahs had all but lost the game too after the Brumbies' reserve hooker Robbie Abel profited from a rolling maul and then Speight won the sprint to his own chip kick to give his side a 21-7 advantage.

A maiden Super Rugby try to replacement half Jake Gordon briefly gave the Waratahs hope before Speight swan-dived over for his second five-pointer to put the issue beyond doubt.

Waratahs captain Michael Hooper was "bitterly disappointed" after his side was dominated at the set piece once again.

"I thought we were the better team in the first half," Hooper said.

"Then sustained pressure again kills us - mauls, scrums. We can't relieve pressure off ourselves and teams like the Brumbies, who are great in that area, are good at grinding you down and they did that."

While the Brumbies are sitting pretty atop the Australian conference, three defeats in a row leave the Tahs languishing near the bottom of the ladder ahead of next Saturday's derby with the Rebels in Melbourne.