The Western Force completed the Australian derby "slam" and sent the NSW Waratahs' finals hopes into a nosedive with a plucky 15-14 Super 14 victory at the Sydney Football Stadium on Saturday night.

The Force kept their own slim playoff hopes alive with a two-tries-to-one win which capped a 25-16 victory over the Brumbies in round three and a 39-7 mauling of Queensland two weeks ago.

The Waratahs now find themselves out of the top four with a 6-4 record ahead of a bye next week, which will deliver no points, and a tough tour of South Africa, where they now realistically need to win all three games.

The Force's win, in front of a disappointing 22,317 fans, was their first ever over the Waratahs since joining Super rugby in 2006.

"It's a great milestone for the club," Force coach John Mitchell said.

Down 9-3 at halftime, the Perth-based side created the better opportunities and were unlucky to be denied twice by the match officials before crossing twice in the second half through Cameron Shepherd and Scott Staniforth.

The Force appeared to be robbed by an over-zealous touch judge in just the 11th minute when Staniforth was ruled to have put a foot into touch as he kicked for Drew Mitchell to spectacularly regather and cross the line. Replays suggested otherwise.

Four minutes after halftime, Richard Brown crossed the line only to be called back for a forward pass from halfback Josh Valentine which also looked good.

The Force were third-time lucky in the 50th minute when, from a quick tap, Valentine deftly sent through a high-bouncing grubber which Shepherd regathered to dive over.

The Waratahs hit back within three minutes when controversial halfback selection Brett Sheehan set up a try with a wide pass to winger Lachie Turner, who popped a pass back for Will Caldwell to celebrate becoming NSW's most-capped lock.

Staniforth completed a slick backline move in the 66th minute and the Waratahs became the victims of their poor execution in 2009 with dropped balls in attacking positions from Dean Mumm, Kurtley Beale and, after the bell, Tatafu Polota-Nau.

Earlier, NSW five-eighth Daniel Halangahu had booted penalty goals in the 16th, 20th and 28th minutes as the home side went to the break with the steadier showing from their raids into opposition territory.

The Western Force have completed the Australian derby "slam" and sent the NSW Waratahs' finals hopes into a nosedive with a plucky 15-14 Super 14 victory at the Sydney Football Stadium.

The Force kept their own slim playoff hopes alive with a two-tries-to-one win which capped a 25-16 victory over the Brumbies in round three and a 39-7 mauling of Queensland two weeks ago.

The Waratahs now find themselves out of the top four with a bye next week - which will deliver no points - and a tough tour of South Africa, where they now realistically need to win all three games.

The Force's win was their first over the Waratahs since joining Super rugby in 2006.

Force captain Nathan Sharpe said the "slam" was not something his side could dwell on with four weeks remaining in the regular season.

"We're still mathematically alive so until we're mathematically dead in the race then we won't dwell on that at all," he said.

Mitchell was less upset at the disallowed tries - and the loss of a bonus point they resulted in - than the policing of the breakdown in the game.

"We tried to play football and often at times we were denied," he said.

"That's probably the most boring game of football that I've seen in a long time in SANZAR rugby and the breakdown was an absolute mess."

Exasperated Waratahs skipper Phil Waugh could barely contain his frustration at another succession of errors from his side.

"Everyone's going to criticise us for making errors and not playing the most attractive footy but, geez, I tell you what, we created a lot more out there than they did, we just couldn't execute," he said.

"We're in the 10th round and we've done the same things every week.

"We lose by a point and, geez, how many points did we leave out on the field?"

On the injury front, Waratahs prop Dan Palmer suffered spasms in his lower back.

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