The benchmark Brumbies have cemented their place in the Super Rugby Pacific top four and likely ended the NSW Waratahs' finals hopes with a water-logged 29-21 derby win in Sydney.

The ACT outfit resisted a spirited second-half fightback from the injury-ravaged Waratahs to not only hang on but also deny the hosts a desperately needed bonus point on Saturday night.

A 78th-minute try from Brumbies replacement forward Sefo Kautai left the Waratahs with nothing tangible to show for their undeniable efforts - and languishing in second-last spot on the ladder.

While the Brumbies are assured of finals football, Darren Coleman's Waratahs side will need to win their remaining three matches to scrape into the playoffs.

"To win in these conditions, it was different," Brumbies halfback Ryan Lonergan said.

"We lacked a bit of effort in that first half and they probably got the better of us.

"I'm pleased we could finish that out there at the end. We got a lot of pay out of our set piece and we played wet-weather footy, sort of grind that one out."

The Brumbies enjoyed all the early running and earned a 7-0 head start with a ninth-minute try to No.8 Charlie Cale.

Already without six front-rowers, the Waratahs suffered another hammer blow in the shadows of halftime when prop Harry Johnson-Holmes limped off with a leg injury.

But, refusing to go away, the Waratahs struck back shortly after through inspirational halfback and captain Jake Gordon.

For all their possession and territory, only a late penalty goal from Ryan Lonergan earned the Brumbies a three-point halftime buffer.

Tries to Wallabies pair Len Ikitau and 2023 John Eales medallist Rob Valetini looked to have given the Brumbies an unassailable 22-7 lead.

But, despite the deplorable conditions, the Tahs hit back with tries to centre Joey Walton and five-eighth Tane Edmed to narrow the deficit to a single point.

Kautai, though, had the final say as the Brumbies notched up a 13th straight victory over the Waratahs in what has become a one-sided arch-rivalry.

"We've got to stay at it. The comp's so tight," said Waratahs milestone man Jed Holloway after having his 100th match for NSW spoilt by defeat.

"We've got to win our next three on the trot, which is do-able.

"We've got to show up, have a good, hard look at this game - look where we need to improve and really build up some of the rugby we showed tonight."