A critical incident has been declared after a 60-year-old man died in hospital following his arrest for allegedly assaulting a woman.

Police and paramedics were called to Lurnea in Sydney's southwest on Friday afternoon after reports of a domestic assault.

A 58-year-old woman was treated at the scene and taken to Liverpool Hospital with serious head injuries.

NSW Police Acting Assistant Commissioner Trent King said a weapon was found at the scene of the alleged attack.

A man known to her who had fled the scene was located a short time later in a nearby suburb, he told reporters on Saturday.

The man was treated by paramedics at an address on Spica Street in Sadleir before being taken to Westmead Hospital, where he died a short time later.

Mr King said the man's condition had deteriora

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