A man has died after an assault at Whangarei Hospital, police confirmed.

The man was a patient in the mental health unit, and had been restrained by police and hospital staff after assaulting a staff member.

Police said they were at the hospital to deal with an unrelated incident.

The man became unresponsive and CPR was carried out but the man died shortly after midnight.

Acting Detective Inspector Lloyd Schmid from Northland CIB said police were investigating.

"A scene examination is currently underway and as part of our inquiries, police will be talking to those involved at the scene as well as any witnesses to this incident to establish what has occurred," Schmid said.

A post-mortem would take place tomorrow, police confirmed.

Northland DHB's head of mental health and addiction services, Ian McKenzie, said police, security and clinical staff were in attendance.

"An external investigation will be undertaken and the death has been referred to the coroner who will determine the cause of death.

"Our thoughts are with members of the whānau at this time."