Jack Ziebell had been celebrating his final match at a function earlier in the evening, his club North Melbourne said in a statement.
He and some teammates staff headed to a South Yarra bar afterwards.
The club said Ziebell was “set upon” when he left the bar by a group of men in an “unprovoked attack”.
“He suffered injuries to his face and head and needed to spend the night in hospital,” the club said.
Police confirmed they were investigating an alleged assault on a 32-year-old man in South Yarra at about 1am overnight.
They said it was believed to be an “unprovoked incident” and the man was not seriously injured.
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They said it appeared the victim was pushed onto the road by one man, who had two others with him.
The victim was allegedly punched in the face and kicked, and fell to the ground, after which the three men fled the scene.
Police are investigating whether the assault may have followed a reported “push and shove scuffle” inside the venue, which saw the three men get kicked out.
Ziebell played 279 matches for North Melbourne between 2009 and 2023, and served as club captain from 2017 to 2022.
His last match was yesterday.