Irish man charged in NSW one-punch attack

A man accused of a one-punch attack that's left his brother fighting for his life has been charged with recklessly causing grievous bodily harm.

The 33-year-old Irishman is accused of knocking his 31-year-old brother onto a Sydney footpath about 3am on Saturday, three days into their Australian holiday.

Paramedics treated the younger man before taking him to St Vincent's Hospital where he underwent emergency surgery and is in a critical but stable condition.

The older sibling has now been charged, police said.

"He has been refused bail and is due to appear in Parramatta Local Court today," the spokeswoman said.

Earlier, police said CCTV footage showed the brothers arguing in Bayswater Road, Potts Point, before there was a push and then a punch thrown.

The 31-year-old man fell backwards and hit his head on the ground.

Their father was in shock and keeping a vigil at his son's hospital bedside, police said.

"The pain in his eyes, you could not describe," Commander of Kings Cross Police Superintendent Michael Fitzgerald told reporters.

"This is a tragedy.

"We have a father who is facing the reality of one son facing serious criminal charges, another son fighting for his life."

The injured man's condition would not be known for the next 24-48 hours, Supt Fitzgerald said.

He said the assault had nothing to do with lockout laws and alcohol was not an aggravating factor.

"This is a family tragedy," he said.