One brother in surgery, another under arrest after alleged one-punch attack in Sydney

A man is undergoing emergency surgery after allegedly being attacked by his own brother in a one-punch attack in Sydney's Potts Point this morning.

Shortly after 3am, a 32-year-old man was allegedly punched in the head on Bayswater Road, and struck his head on the pavement when he fell.

He was treated at the scene by paramedics before being taken to St Vincent's Hospital where he is now undergoing surgery in a critical condition.

His 33-year-old brother is assisting police with their inquiries.

When police arrived, the brother was allegedly kneeling down with blood on his hands, visibly upset.