A 10-6 victory in the Canberra Raiders Cup preliminary final ended the Queanbeyan Blues' three-peat ambitions and with it any dreams of a grand final local derby.

West Belconnen's win guided it into the big dance for the first time in nine years against the flying Queanbeyan Kangaroos, who haven't dropped a game since their season opener.

With the Kangaroos watching on from the hill, at times it seemed like neither side wanted to win as the error count soared at Seiffert Oval.

Thompson praised his side's effort in defence after grinding out a "pretty ugly" win.

"It was a funny funny game," Thompson said.

"The game was probably there for the taking for both sides. [It was] a real good effort from the boys, they just keep turning up in defence but we just keep turning the ball over.

"If we can hold the footy and play some good structure, I think we'll be a chance [in the grand final]."

The emotion-charged Blues — donning black armbands in honour of front-rower Trevor Thurling's late mother — opened the scoring through five-eighth Luke Branighan in the third minute.

West Belconnen struck back seven minutes later as hulking centre Tevita Aroha shrugged off three defenders to open his side's account, and for some time it looked as though that would be it.

The Blues would make a bust and turn the ball over, before West Belconnen would return the favour as the game developed into a dour struggle.

For much of the second half it seemed as though a Josh Baker conversion would be enough to send the Blues into their fourth straight grand final.

That is until Warriors fullback Ryan McQueen sliced through the defence and slammed the ball down to push West Belconnen in front with 12 minutes left.

McQueen could have a big role to play in the decider, with halfback Cody Wynnik set to be ruled out with a concussion.

Thompson said Wynnik's second concussion in as many weeks and "probably his fourth or fifth this year" raised the red flag.

"For a duty of care we've got to [look after him]," Thompson said.

"We're not going to risk a player's health for a game of footy."

The Warriors are hopeful Raiders under-20s player Andre Niko will return at fullback, shifting McQueen into the halves.

Thompson said the Warriors would have "nothing to lose" as they went in search of grand final success next Sunday.

"We've been winning ugly, I don't know whether that's a good thing or a bad thing though," Thompson said.

"The Roos have been an outstanding team all year, so we've just got to come here, show up and just play our footy, and have a real good crack at it."

WEST BELCONNEN WARRIORS 10 (T Aroha, R McQueen tries; R Roberts goal) bt QUEANBEYAN BLUES 6 (L Branighan try; J Baker goal).