THE AFL has ticked off a contentious umpiring decision that cost Nick Watson a goal during Hawthorn's win over St Kilda on Thursday night.

Watson's shot from an acute angle at half-time was disallowed after the umpire ruled that he had run off his line when he took the kick, meaning the score did not count because he had effectively played on after the siren.

On Friday, the League backed umpire Brent Wallace's call, saying the Hawks gun forward had deviated slightly as he was running into goal. 

"The umpire, positioned directly behind the kick, correctly disallowed the score after Nick Watson moved off his line to the right and didn't kick the ball over the man on the mark," the AFL said in a statement.

"The AFL have contacted Hawthorn and St Kilda today to provide an explanation regarding the decision."

Following Thursday night's match, Hawks coach Sam Mitchell said the rule didn't make sense and that the club would seek clarification from the League.

"We'll certainly be going to the AFL to ask about it … as a right-footer, there is absolutely no reason that a player would go wider to give themselves an advantage, and that's what the rule is there for," Mitchell said.

"The vision doesn't look like he goes off his line much, but that's the umpire's call and I can accept that. But the fact that you can go off the line towards the boundary and it be called play on ... there's no common sense about that.

"When something doesn't make sense, they fix it pretty quickly. Why would he run wider to give himself a harder shot and it get called play on? Didn't make a lot of sense."

Watson was also unimpressed that the goal was disallowed.

"Don't get me started … they're pretty keen to get the whistle out," Watson told Fox Footy.

"Actually I won't say anything about the umpires, I might get a fine."

The Hawks dominated all night, holding the Saints goalless in the first half before cruising to a 52-point victory.