A Henry Speight brace has kept the ACT Brumbies' Super Rugby  finals hopes alive, but it could have come at a massive cost with a potentially season-ending knee injury to Wallabies lock Sam Carter.

Speight ended his try-scoring drought to score two brilliant tries in a 22-16 victory over the Pretoria Bulls in front of 9799 fans at Canberra Stadium on Friday night.

Brumbies coach Stephen Larkham is confident Carter hasn't injured his anterior cruciate ligament and thinks the damage might be limited to his medial, but he won't know until they get results of Saturday's scans.

The win puts the Brumbies on top of the Australian conference by just one point until at least Sunday morning when the NSW Waratahs play the Johannesburg Lions in South Africa.

But Carter looked to have done some major damage to his knee when Bulls winger Njorn Basson was tackled  and cannoned into the Wallabies lock's knee half way through the first half.

Carter's knee buckled at an alarming angle and he lay on the ground in agony for a long time, before getting it strapped up and trying to run it off.

But he immediately headed to the bench with a problem that could have ramifications for not only the Brumbies' finals hopes, but the Wallabies' World Cup ambitions.

Larkham was confident it wasn't an ACL.

"We don't know how bad it is yet, he'll have  a scan, he's in a brace at the moment," he said.

"It doesn't look like good news at this stage, we'll wait and see what the scan reveals. The only thing I heard was it was just a medial."

Speight's brace split the game wide open in the second half and Bulls coach Frans Ludeke said those efforts were the difference between the sides.

The Fijian flyer started with a brilliant, long-range solo effort off the Brumbies scrum, fending and zig-zagging his way through the Bulls line and then using his fend to perfection for his second to cap off a lovely Brumbies build-up.

It took the Brumbies from a 9-8 deficit to a commanding 22-9 lead.

But they had to endure a nerve-wracking final 13 minutes after the Bulls' maul finally got going and Pieter Labuschagne capitalised by diving over to give the visitors hope.

The Brumbies held on, their dominant breakdown work and ability to mostly negate the Bulls' maul helping them despite some nervous moments in the dying minutes.

Ludeke said the Brumbies were the complete package and definitely Super Rugby title contenders.

"[They're] the best defensive outfit, they score a lot of tries as well so the balance is there," Bulls coach Frans Ludeke said.

"They've got big-match temperament, so when the big games come up like tonight they know how to guts it out and play it smart, play field position, use the opportunities ... definitely a huge contender for the competition."

rumbies flyhalf Christian Lealiifano's kicking woes continued. He missed two early penalties, which meant the hosts weren't able to take advantage of their momentum, but he did kick a crucial conversion from out wide after Speight's second try.

Larkham said he was a quality kicker and they would continue to back him in.

Bulls flyhalf Handre Pollard was on song with his boot and he made the Brumbies pay early. He slotted two shots from either sideline to put the visitors up 6-0 after 22 minutes.

Known for his work at the breakdown, David Pocock continues to show his worth as an offensive weapon - not just from a maul, but his running play as well.

He created a half-break for centre Robbie Coleman, who was brilliant in the No.12 jersey.

Nic White got a quick recycle and  found Jordan Smiler - on for the injured Carter - out wide with a cut-out pass with the tryline begging. Smiler fended and charged over for the first try of the game.

The Brumbies now have a tricky trip to Perth to face the lowly Force. It's a fixture they've struggled with in recent years.

They then face an even tougher match against Super Rugby powerhouse Canterbury Crusaders in the final round of the home-and-away season.

ACT BRUMBIES 22 (Henry Speight 2, Jordan Smiler  tries; Christian Lealiifano penalty, 2 conversions) bt PRETORIA BULLS 16 (Pieter Labuschagne try; Handre Pollard 3 penalties, conversion) at Canberra Stadium. Referee: Marius van der Westhuizen. Crowd: 9799.