A man is in a critical condition after being punched in the head in the early hours of Saturday morning.

Police say the 21-year-old fell to the ground and lost consciousness after he was allegedly punched in the face outside a house party in Minchinbury.

He was rushed to Westmead Hospital with head injuries and remained in a critical condition on Saturday night. 

A 21-year-old was arrested at the scene and has been charged with several offences including causing grievous bodily harm with intent.

Emergency services and police were called to the home on Rupertswood Road following reports of the assault.

Witnesses told police the incident occurred after the alleged attacker argued with his girlfriend at the party.

He then allegedly struck the victim who he was talking with outside the house.

The attack follows a spate of one-punch assaults which prompted the introduction of a new offence for hitting a person and causing their death, with increased penalties if the assault was fuelled by alcohol.

The government also introduced early closing and 1.30 am lock-out laws in entertainment precincts following the death of 18-year-old Daniel Christie, who has hit in the head on New Year's Eve in Kings Cross.  

The alleged offender will face Parramatta Bail Court on Sunday.

Police are urging anyone with information about this incident to call Crime Stoppers on 1800 333 000