TWO key second-half moments saw a Western Force dominance fade and with it the chance to go top of the National Rugby Championship in the 29-19 loss to Canberra Vikings at Viking Park.

The Force had gone into the break with a two point lead and overwhelming possession and territory coming from the work of their forwards.

But an hour into the game, as they pressed to extend the lead, they were out-scrummed twice on the Vikings line and fortunes turned as the impact of the strong home reserves bench exerted its influence.

The Vikings went on to score two tries to add to their first-half pair while the Force were held scoreless after running in three five-pointers before the break.

The Force, who slipped from second to fourth on the ladder, cannot afford such a shift in momentum when they take on leaders Queensland Country on the Gold Coast on Saturday.

And they will need to work on body height going into contact. In too many crucial moments they allowed defenders to hold them up and gain turnovers.

It was not the return to Canberra that Force coach Tim Sampson, who took the Vikings to last year's grand final, was hoping for as his side had a seemingly good five-pointer disallowed and bombed another when full-back Jack McGregor dropped the ball as he tried to ground it.

Sampson knew he was in for a battle against a home side that contained players returning from Wallabies camp, 12 Super Rugby Brumbies in the starting side and another four in the reserves.

"We set ourselves up nicely for the second-half but just couldn't get over the line," Sampson said.

"There were three or four, back-to-back, crucial errors and the contact area is a big one for us. Their bench certainly strengthened the scrum.

"An opposition with 16 contracted players will take advantage when you make mistakes.

"To have been ahead at 60 minutes was a great effort but we've got to kick on and put opposition away."

Flanker Chris Alcock, centre Henry Taefu and No.8 Brynard Stander crossed for the Force with captain Ian Prior adding two conversions.

Vikings winger Andy Muirhead grabbed a double, and centre Jordan Jackson-Hope crossed with fly-half Wharenui Hawera who also kicked three conversions and a penalty.