The league has released video of the frenetic last few seconds in Saturday night's match at Optus Stadium.
North defender Daniel Howe cleared the ball out of defence with a long kick and just as the ball was going out of bounds, the final siren sounded.
As Fremantle players and fans appealed for a deliberate out of bounds free kick, the umpires immediately ruled that the siren had sounded before the ball had crossed the boundary line.
There was less than one second in it.
If the Dockers had been awarded the free, they would have had a shot on goal from about 45m on the boundary to win the game.
"The final siren sounded before the ball goes out of play. The umpire's decision on the ground was correct," the league said in a statement.
The AFL's video included audio from the umpires.
"The siren went before (the ball went out of bounds), that's the call we make," one umpire is heard saying.
Dockers players then appealed to the umpires for a free, but the decision stood.
Fremantle coach Justin Longmuir said post-match he trusted the umpire "got it right".
"I'm not 100 per cent sure on the rules. You know what, I trust the umpire got it right," Longmuir said.
Fremantle have been tipped widely to make the finals again this season, but are winless are two rounds, while the Kangaroos under Alastair Clarkson are unexpectedly 2-0.
Fremantle were also involved in Sirengate, the 2006 controversy at the end of their round-five match against St Kilda in Launceston.
The umpires failed to hear the final siren and the Saints then kicked a point to draw the game.
The league investigated and, after a four-hour hearing a few days later, ruled that Fremantle should be awarded the win.
It was only the second time in VFL/AFL history that the score and result of a match were changed on protest.
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