Officers arrived at a home in Casino, in the Northern Rivers region, about 2.27am on Saturday after receiving a call over welfare concerns.
They found a woman, who is yet to be formally identified but thought to be in her 40s, with "obvious injuries" to her head caused by a physical assault, NSW Police Assistant Commissioner Peter McKenna said.
"She was unconscious but breathing," he told reporters on Saturday.
"Police assisted her and called for an ambulance immediately (which) arrived very shortly after but unfortunately her condition deteriorated and she died at the scene."
A 31-year-old man was arrested at the scene and taken to Lismore Police Station, where he was subsequently charged with one count of murder.
He is believed to have been in a relationship with the woman for some months, Mr McKenna said.
A crime scene has been set up and a regional critical incident team will investigate all circumstances surrounding the case, including the police response.
A triple-zero call was made just after 1.30am but it took until 2.25am for police to acknowledge the call, with officers in Casino arriving at the scene two minutes later, Mr McKenna said.
"The delay in the timing of police acknowledging that call and attending the scene has given me enough concern that I've asked for an independent review of this investigation as to what that delay was and if it was justified," he said.
"We want to see what happened from the time that call was made, how the radio operator dispatched that call, the circumstances around the timings till it was acknowledged and until police attended."
The independent critical incident probe will look at whether the call was allocated the right priority and if it was acknowledged as such.
Homicide squad members are separately working with local officers to investigate the woman's death.
Mr McKenna said the force takes domestic violence very seriously and the issue is at the "top of our priority list".
"It is one of the most serious crimes there is and we will do everything we can to take this as seriously as we can and make sure people are held to account and put before the courts," he said.
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