AFL umpires coach Hayden Kennedy has conceded that the contentious free kick that led to Allen Christensen's game-winning goal in Friday night's Carlton-Geelong duel was a mistake.

Christensen was paid the pivotal free kick at the 23-minute mark of the final quarter, with the overruling umpire coming into the fray to award a push in the back against Blue Sam Docherty.

Umpire Brendan Hosking made the decisive call from further away than the experienced Brett Rosebury and Kennedy said play should have been allowed to go on.

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"We've had a pretty strong look. We understand everything that went through the mind of Brendan (Hosking), where the ball was and where the fist was in comparison," Kennedy told 3AW on Saturday.

"Ultimately we've decided that it was an unwarranted free kick."

Kennedy said there was some confusion about the adjudication of the push against Docherty, rather than an 'arm chop'.

Kennedy said it was imperative the umpires backed themselves to pay the free kicks when they are there.

But on Triple M he reiterated that out of zone free kicks must be "clear and obvious". 

"Sometimes due to body positions or bad angles the controlling umpire can't see and that's where the other umpire comes in," Kennedy said.

"We've moved them (umpires) closer this year because we know the congestion is really high."

Blues coach Mick Malthouse said after the game he found the decision "a bit strange".

The six-point loss to Geelong followed a remarkably similar pattern to the five-point loss in the teams' round 12 clash this year.

In that game the Blues were left to rue another close loss after forward Troy Menzel was denied what looked a certain free kick for high contact in the dying seconds.

Umpires boss Wayne Campbell admitted the day after the game that Menzel should have received a free kick.