It was a horror end for Canberra as Penrith piled on a hat-trick of tries to rob the home side of its 18-16 lead with nine minutes remaining to win the match 34-18.

There were sighs of relief from referees, who could face sanctioning after it was revealed the video referee incorrectly ruled on a knock-on and disallowed a legal try.

For most of the match it looked like the decision to deny Penrith the try could also cost it the game until a length of the field try by winger Amos Roberts in the 71st minute turned things around.

It was the start of a spectacular ending for Penrith, with winger Luke Rooney posting tries in the 75th and 77th minutes and Ben Ross finalising the Panthers' scoreline with their seventh try in the dying seconds.

But it should have been eight tries for the visitors.

Touch judge Russell Turner spotted a knock-on well before Panthers backrower Tony Puletua crossed the tryline in the 35th minute.

Referee Sean Hampstead was ready to award the four points until he was alerted to a possible knock-on by Turner.

Video referee Phil Cooley intervened and confirmed lock Trent Waterhouse had knocked on in the play prior to the one that ended in Puletua crossing the line.

NRL chief operating officer Graham Annesley said it was not within Cooley's jurisdiction to rule on the previous play.

"I've not seen it but as it's been described to me that's not part of the current policy," he said.

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