The Bulls have suffered a sixth Super Rugby defeat in 13 matches this season. They are now also in danger of being knocked off the top of the South African Conference by the Jaguares or the Sharks.
The Brumbies scored four tries to one to beat the Bulls 22-10 in Canberra after leading 12-10 at half-time.
Outside centre Tevita Kuridrani scored a hat-trick of tries to put the Brumbies on course to return to the play-offs bracket when they beat the Bulls in Canberra.
The Bulls on the other hand drop down to second place in the South African conference behind the Jaguares, while the Sharks can also leap-from the Bulls should they beat the Lions in Durban on Saturday.
The Bulls came into the fixture full of confidence after their winning performance against the Rebels in their tour opener last Friday.
They were facing a Brumbies side that returned from a bye, looking to make it six home wins in a row.
The Brumbies scored the opening try through their big and strong backline players — running lines and angles that had the Bulls defence in sixes and sevens.
Kuridrani was in for his first and the Brumbies took the lead following an earlier Manie Libbok penalty.
The Pretoria outfit took advantage of a 14 man Brumbies team after Thomas Cusack was sin binned. They forced their way over from a scrum for Hanro Liebenberg to dot the ball down for a converted try.
But the Brumbies pilled on the pressure and two minutes before the break, they again went wide as Kuridrani sliced through a gap in the defence to go over for his second.
Christian Lealiifano converted for a 12-10 halftime lead.
In the second half, the Brumbies were quick off the mark, taking the opportunity of the scrum in an attacking position.
Henry Speight set-up Irae Simone for a try following a Lealiifano inside pass.
Kuridrani got his hat-trick as the Bulls once again allowed the Brumbies to go wide and open up a 12 point gap that sealed victory.
The win for the Brumbies sees them remain top of the Australian conference.
In Hamilton, the Chiefs scored three tries to one for a 19-13 victory over the Reds. All three tries were scored in the first half that saw the home-side take a 19-6 halftime lead.
This result momentarily moves the Chiefs up to 10th place on the standings, while the Reds move down to 14th.
Meanwhile the Springboks Sevens team will have an opportunity this weekend to secure their Tokyo 2020 Olympic Games qualification.
The London leg of the series is the penultimate of a season where the top four are guaranteed a place at the Olympics.
The Blitzboks are currently in fourth place on 121 points following their recent two tournament wins.