A woman has died at Auckland Region Women's Corrections Facility.
The woman, who was a prisoner, was found in her cell at about 4.20am.
Prison director Steve Parr said staff immediately called for an ambulance and a nurse.
"[The staff] began CPR. Despite all efforts, staff were unable to resuscitate the woman."
Staff had been carrying out a welfare check on the woman at the time.
Parr said the death was not suspicious.
"It appears the woman died of natural causes as a result of a pre-existing medical condition. Prisoners and staff affected by the woman's death are being provided with support.
"Our thoughts are with the woman's family and friends."
All deaths in custody were referred to the Coroner and there would also an investigation by the independent Corrections Inspectorate, Parr said.
Police would notify the woman's next of kin.