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 60-year-old 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 deteriorated soon after he was arrested.
"The information I have at this stage is that the man was handcuffed for a short period of time," he told reporters at Penrith.
"Then as his health declined, the handcuffs were removed at the request of NSW Ambulance so they could continue their treatment.
"There was no force from the information that I have."
Mr King said he stood by the actions of the officers involved.
The independent police watchdog is investigating the incident as a death in custody, with a report to be prepared for the coroner.
In another alleged domestic violence incident, a 29-year-old man has been charged with murder after he was arrested and taken to Byron Bay Police Station.
He has been refused bail and will appear before Parramatta Bail Court on Sunday.
The Mullumbimby man was arrested overnight after a 23-year-old woman was found dead inside a unit on Poinciana Street, Mullumbimby.
Police were called to the home in northern NSW after reports of a domestic incident involving the woman, who is yet to be formally identified.
More than 38,000 domestic violence assault incidents were reported to police in NSW between July 2024 and June 2025, according to the state's Bureau of Crime Statistics and Research.
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