An inmate at an Auckland prison has been found dead in her cell overnight.

Corrections confirmed a woman in custody had died at the Auckland Region Women's Corrections Facility in Wiri, South Auckland.

Prison director Steve Parr said staff on site were carrying out a welfare check just after 4.20am when they found the inmate unresponsive in her cell.

"Our staff immediately called for an ambulance and a nurse and began CPR.

"Despite all efforts, staff were unable to resuscitate the woman."

A St John spokeswoman confirmed that a rapid response ambulance was called to Wiri at 4.38am.

Paramedics were called to a medical emergency, she said.

Parr said police were also called and would notify the woman's next of kin.

"While investigations are under way, the death is not suspicious and it appears the woman died of natural causes as a result of a pre-existing medical condition."

He said staff and prisoners affected by the death are being provided support.

"All deaths in custody are referred to the Coroner for investigation and determination of cause of death," he said.

"For all deaths in custody, there is an investigation by the independent Corrections inspectorate."