Western Force's winless start to the Super 14 season continued as the Bulls came from behind to post a 28-15 triumph in Perth on Saturday night.

James O'Connor nailed five penalties as the Force edged ahead 15-10 in the 49th minute but the defending champions piled on the final 18 points of the match to ensure their unbeaten run this season extended to six games.

No.8 Pierre Spies, fullback Zane Kirchner and five-eighth Morne Steyn all crossed but the Bulls couldn't find a fourth try that would have earned them a bonus point.

The Force's sixth loss on the trot keeps them rooted to the bottom of the table on just one point, with even the hapless Lions (three points) above them.

The top-of-the-table Bulls shoot five points clear of NSW, Crusaders and the Stormers, who are all locked on 23 points.

The Bulls attacked relentlessly from the outset and were camped inside the Force's 22m for much of the opening nine minutes before Spies crashed over the line.

Steyn's conversion made it 7-0 but two penalties from O'Connor reduced the gap to one after 20 minutes despite referee Steve Walsh continually penalising the home side.

Steyn's first penalty of the night in the 34th minute made it 10-6 but the Force finished the half in a flurry, with another two penalties to O'Connor, the last bouncing in off the upright, giving the home side a 12-10 lead at the break.

Force winger Scott Staniforth had a productive first half while O'Connor was also electric in attack.

Handling errors cost the Bulls dearly in the first 40 minutes and their fumbles continued early in the second, wasting good field position.

The Force were more clinical in their movements forward and O'Connor's fifth penalty of the night gave the home side a handy five-point buffer.

But from there it was all one-way traffic, with Steyn starting the Bulls' run with a try in the 57th minute after shaking off a weak tackle from Ryan Cross.

Steyn's conversion made it 17-15 to the Bulls and the Force again started falling foul of the law, with Steyn's second penalty of the night in the 63rd minute making it 20-15.

And it became one too many offences for the Force two minutes later when Cross was given his marching orders.

Kirchner made the most of the numerical advantage, finishing off a surging run from substitute Flip van der Merwe to seal the win.

"It was a step up from last week," Force coach John Mitchell said.

"We were just a little bit untidy post half-time getting it out of the clearance area. But this group fought extremely hard tonight."

Bulls coach Frans Ludeke praised his team's fighting spirit.

"We were really happy with the win," Ludeke said.

"The first half we got in the right areas, maybe didn't use all our opportunities but second half we were clinical, got up there and got the points.

"I think it was just patience, just kept applying the pressure."

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