The Brumbies have clinched the Dan Vickerman Cup with a watershed 28-12 Super Rugby victory over the NSW Waratahs in Sydney.

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

The four-tries-to-two triumph was also the Brumbies' record ninth straight win against Australian opposition, leaving the ACT outfit top of the conference after a week in which their very existence came into question.

But it was a forgettable night for the Waratahs, especially flanker Jack Dempsey, who 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.

Without Wallabies playmaker Bernard Foley for the fourth straight week, the Waratahs still managed to draw first blood, with prop Tom Robertson burrowing his way over in the ninth minute.

Reece Robinson's conversion gave the Tahs a 7-0 lead, but the sides went to the break all tied up after Brumbies centre Tevita Kuridrani powered over under the sticks 11 minutes before the break and Wharenui Hawera converted.

Once again faltering at the set piece and handing over possession as they did in their two defeats in South Africa, the Waratahs lost their way in the second half and now find themselves languishing near the bottom of the ladder.

Waratahs captain Michael Hooper was unable to disguise his how upset he was after his side was dominated at the set piece.

"I'm bitterly disappointed with that," he said.

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