A man has been arrested over an alleged one-punch attack, which left another man in a serious condition, in Sydney's east.

It's alleged the 24-year-old who had been playing the pokies earlier in the night then struck a 23-year-old man outside the Matraville Hotel around 1.30am this morning.

Paramedics were called to the scene and the man was taken to Prince of Wales Hospital with a suspected fractured jaw.

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The 23-year-old is now undergoing surgery with fears of permanent brain damage.

Detective Acting Inspector Michael Capon said the alleged attack was "unprovoked" but that the two men had had a disagreement earlier in the night.

"We have information that there was a confrontation earlier in that evening between two males within a licensed premise," he said.

"And then there was a later confrontation out on the street on Bunnerong Road that morning."

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Capon said the man appears to have suffered injuries from being punched and then hitting the footpath.

"These one-punch style hits are extremely dangerous to the victims involved," he said.

The 24-year-old was arrested at a home in Maroubra this afternoon.

He was taken to Maroubra Police Station where charges are expected to be laid.