After originally backing the verdict on Saturday night, the league now has conceded the wrong decision was made and the goal should not have been allowed.

It is a massive hit to the credibility of the league's new ARC, or AFL review centre, which was opened at the start of the finals.

One of the key functions of the centre was to streamline the contentious video reviews of scores and help ensure greater accuracy.

But as Saturday's near-disaster showed, there is still potential for human error, however well the review process is followed.

Thomas snapped a goal to bring Collingwood within 20 points of GWS in the final quarter of the pulsating preliminary final on Saturday at the MCG.

While no umpires called for a review, it was looked at by the video centre as a so-called "silent review".

Replays clearly showed the ball was touched off Thomas' boot.

But the goal was allowed to stand.

It would have been a massive controversy had Collingwood won.

But the Giants saved the AFL's blushes when they hung on to win by four points.

"The AFL acknowledges that upon review the ball was touched by GWS player Lachie Keeffe," the league said in a statement.

"In determining a decision, the reviewer must be clearly satisfied that there is sufficient evidence beyond reasonable doubt to overturn a decision.

"On this occasion, the reviewer at the time was not convinced the ball was touched beyond reasonable doubt to intervene and overturn the decision of the umpire.

"This is in line with the correct process for a silent review."

Australian Associated Press