An Irishman is reportedly in critical condition and undergoing emergency surgery in Sydney, Australia after allegedly being punched by his brother.

Shortly after 0300 local time on Saturday (1600 Friday, Irish time) New South Wales police were called to the Bayswater Road area after reports of an assault.

Police were told that a 33-year-old man punched a 32-year-old man in the head, causing him to fall onto the footpath.

The Sydney Morning Herald reports that the men were Irish brothers, in Australia on holiday with their father.

"This is a family tragedy," said Commander of Kings Cross Police, Superintendent Michael Fitzgerald. "This has nothing to with the lockout [laws] or early closing times."

Supt Fitzgerald said alcohol was not a factor in the alleged assault.

The younger man sustained serious head injuries and was treated by paramedics before being taken to St Vincent’s Hospital.

The 33-year-old was arrested and is assisting police with their inquiries at Kings Cross Police Station.

The scene at Bayswater Road is being examined by police.