A MISSED 50m penalty in the dying stages of Greater Western Sydney's preliminary final loss might have cost the Giants their maiden Grand Final appearance, Laws of the Game committee member Michael Christian says.

Tom Scully marked the ball in the centre square with just under two minutes left and Western Bulldogs defender Jason Johannisen entered Scully's 10m protected zone.

GWS trailed by five points at that stage and Scully booted the ball into the forward line.

The Dogs didn't allow the Giants to score for the rest of the game and held on for a six-point win to send them into their first decider since 1961.

If Scully had been paid a 50m penalty, he would have had a set shot at goal within scoring distance.

WATCH: The thrilling final minutes of GWS v WB

"There was a free kick earlier this season paid against Kieren Jack against Hawthorn, and the umpires deemed that was a correct decision," Christian said on RSN927 radio.

"Well, if that was a correct decision … this definitely should have been paid a 50m penalty."

The 1990 Collingwood premiership player was referring the Sydney Swans' round 17 loss to Hawthorn.

Jack was penalised for entering Jonathan O'Rourke's protected zone just before the end of the third quarter, and the Hawk capitalised with a goal. 

"Johanissen started probably 10, 11, maybe 12m from behind the mark, but didn't make any attempt to veer out to get out of that protected zone," Christian said.

"Scully was wanting to roll on to his left boot and (Johannisen) impeded his movement."