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 high-flying 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.

The third-placed Brumbies remain firmly on track for another finals series, even if they were happy to take the points and head back to Canberra despite being unsatisfied with their performance.

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

"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."

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.

"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."

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

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

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.

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.

"It's been a disappointing year. I'm not going to sugar coat it," Holloway said.

"But we're still in the hunt - surprisingly - so we've got to stay at it."

The Tahs' late-season draw is favourable, with matches to come against the bottom-placed Western Force, along with Moana Pasifika and Queensland Reds.

The Brumbies host the defending champion Crusaders and Melbourne Rebels before rounding out their finals build-up against the Force in Perth.

Waratahs 21 [Tries: Gordon, Walton, Edmed; Cons: Harrison 2, Edmed] Brumbies 29 [Tries: Cale, Valetini, Ikitau, Kautai; Cons: Lonergan 2, Debreczeni; Pen: Lonergan]

Waratahs: 15 Mark Nawaqanitawase, 14 Triston Reilly, 13 Joey Walton, 12 Lalakai Foketi, 11 Dylan Pietsch, 10 Will Harrison, 9 Jake Gordon (captain), 8 Jed Holloway, 7 Charlie Gamble, 6 Lachlan Swinton, 5 Miles Amatosero, 4 Fergus Lee-Warner, 3 Harry Johnson-Holmes, 2 Jay Fonokalafi, 1 Hayden Thompson-Stringer.
Replacements: 16 Ben Sugars, 17 Lewis Ponini, 18 Pone Fa’amausili, 19 Hugh Sinclair, 20 Hunter Ward, 21 Jack Grant, 22 Tane Edmed, 23 Izaia Perese.

Brumbies: 15 Tom Wright, 14 Andy Muirhead, 13 Len Ikitau, 12 Tamati Tua, 11 Ollie Sapsford, 10 Jack Debreczeni, 9 Ryan Lonergan, 8 Charlie Cale, 7 Rory Scott, 6 Rob Valetini, 5 Nick Frost, 4 Darcy Swain, 3 Allan Alaalatoa (captain), 2 Connal McInerney, 1 Harry Vella.
Replacements: 16 Billy Pollard, 17 James Slipper, 18 Sosefo Kautai, 19 Cadeyrn Neville, 20 Luke Reimer, 21 Harrison Goddard, 22 Declan Meredith, 23 Ben O’Donnell.

Referee: Ben O’Keeffe
Assistant referees: Stu Curran, Jackson Henshaw