A man suffered a fractured skull when he was allegedly attacked with a baseball bat in southern Sydney last night.

Two men, 19, have been charged with assaulting the man following an altercation with him and his daughter.

Neighbours reportedly saw the victim and his daughter running from the twins and heard them yelling "leave us alone", according to 7 News.

Police allege the father, who had been visiting another of his daughters, was assaulted by the twins and a group of three other men, who punched and kicked him as he lay on the ground.

The incident occurred when the man, 42, and his daughter, 20, went to a home in Riverwood to return a mobile phone. While there, the man talked with his other daughter, 19, and the twin brothers, according to police.

The conversation escalated, and the father and his older daughter fled the home. Police allege the twins chased them, one armed with a baseball bat.

The father was then allegedly assaulted by one of the twins at the intersection of Truman Rd and Belmore Rd.

"This guy with a baseball bat just came and cracked him on the back of the head," a witness told 10 Daily.

"During the assault, three other men joined in and punched and kicked the man while he was on the ground," NSW Police said in a statement. Those men fled the scene before police arrived.

Witnesses called emergency services and when they arrived at the scene they found the father unresponsive.

He was taken to St George Hospital and treated for a fractured skull. He is in a stable condition.

The twins were arrested at the scene and taken to Campsie Police Station.

They've been charged with reckless grievous bodily harm in company. Bail was refused and they will appear in Burwood Local Court today.