George Smith and Pat McCabe face stints on the sideline after both injured knees in a heartbreaking 28-22 loss to the NSW Waratahs.

Smith injured his medial ligament and left the ground in a knee brace with the prospect of being out for six-to-eight weeks.

That would rule him out of the British and Irish Lions series and potentially the rest of the Brumbies' campaign. The Brumbies have now won just three of their past eight matches and the form which lifted them to the top of the ladder has disappeared.

It's not quite panic stations. But the Brumbies are now in a three-way dogfight with the Waratahs and the Queensland Reds to clinch Australia's guaranteed finals berth.

The attacking play has evaporated, as has the clinical edge which had many pundits installing them as championship contenders. Coach Jake White now has a mission to arrest the form slide and try to stabilise the club's bid to break an eight-year finals drought. The Brumbies have just four games remaining in their regular season.

They play the Auckland Blues - their 2012 destroyers - the Wellington Hurricanes, Melbourne Rebels and Western Force.

And their depth is set to be tested as they lose players to a 25-man Wallabies squad and injuries set in.

Smith and McCabe were forced from the field in the second half. McCabe is poised to sign a new deal with the Brumbies and the ARU and it's thought his injury isn't as serious. Smith, however, will wait on scan results on Sunday before knowing his immediate future.

The Brumbies were also privately fuming with the way referee Steve Walsh officiated the match.

Walsh was ruthless on the Brumbies, but when they had a chance to win the game with a strong rolling maul which appeared destined to deliver a try, Walsh ruled they collapsed it and gave the ball to the Waratahs.

The Brumbies have a long history with Walsh, dating back to his stoush with skipper Stephen Hoiles in the corresponding match in 2010.

Earlier, the Brumbies looked set to consolidate top spot on the table and end NSW's finals hopes when they led the Waratahs 22-21 as the clock wound down.

But a try to winger Peter Betham six minutes from full-time, after a lovely pass from five-eighth Bernard Foley, saved the Waratahs' season for the second week in a row.

Two long-range penalty goals from Brumbies fullback Jesse Mogg had broken a 16-all second-half deadlock before a disappointing crowd of just 21,817 spectators at ANZ Stadium.

But after putting flanker Michael Hooper over in the 45th minute, Berrick Barnes - who replaced Rob Horne at half-time - combined with Hooper again to score himself on the wrap around to spark the Waratahs' fightback with a quarter of an hour remaining.

The Brumbies led 13-6 at the interval after posting the only two tries of the half through skipper Ben Mowen in the 21st minute and centre Tevita Kuridrani in the 37th minute.

But two unsuccessful conversion attempts from Mogg kept the home team in touch.

Mogg's fullback showdown with Waratahs star Israel Folau ended in a stalemate with Mogg's 15 points from five penalty goals countering another impressive display from Folau in both attack and defence.

Brumbies captain Ben Mowen commended Barnes for ''taking the game by the scruff of the neck'' late on. with AAP

NSW WARATAHS 28 (Michael Hooper, Berrick Barnes, Peter Betham tries; Brendan McKibbin 2, Berrick Barnes penalties; Berrick Barnes 2 conversions) bt ACT BRUMBIES 22 (Ben Mowen, Tevita Kuridrani tries; Jesse Mogg 3, Christian Lealiifano penalties)at ANZ Stadium. Referee: Steve Walsh. Crowd: 21,817.