The West Belconnen Warriors took out the competition to win their first premiership in nine years as they upset the heavily-favoured Kangaroos at Seiffert Oval on Sunday.

A penalty by Bobby Roberts secured the Warriors their grand final win in the final minute of the 26-24 game.

Trailing 18-6 at half-time, West Belconnen fought its way back into the contest to level it up at 24-all with seven minutes left. 

It looked as though the Canberra Raiders Cup grand final would go to extra-time for a fourth consecutive year before a Brent Crisp shoulder charge sent West Belconnen half Cody Wynnik to the turf.

Roberts calmly slotted the penalty goal and somehow the Warriors had found a way to win. 

Roos had a chance to put the game beyond doubt with nine minutes left before halfback Lachlan Cooper opted against a field goal attempt and took on the defence.

Cooper was pulled down on the last tackle, and just minutes later man of the match Tevita Aroha-Tuinauva scored his second try to bring the Warriors within two points. 

Roberts' conversion from the touchline was followed by a bow to the hostile Roos crowd, and the rest was history. 

Les McIntyre Medal winner Roberts struggled to find words to do the feeling justice after snapping the Roos' 16-match winning streak.

"They've had a great season and credit to them, but every side's beatable," Roberts said. 

Shades of the Canberra Raiders were on show when West Belconnen fans erupted into a Viking clap to welcome their side out for the second half. 

West Belconnen coach Troy Thompson said game plans went out the window in the "surreal" dying minutes. 

"Having a shoulder charge is probably a bad way to end for [the Roos] because they've been a great side all year," Thompson said.

"But we've just got to take that. It's unbelievable."