A Police investigation is underway into how a man died in the cells of an Auckland court this afternoon.

Police say the 36-year-old was alone in the cell at Papakura District Court when he was found unconscious at 3.45pm.

Officers and paramedics attempted CPR but he wasn't able to be revived.

The man was in custody for breaching bail on charge of kidnapping, injuring with intent and assaulting a female.

A post-mortem will be carried out to establish cause of death.

The incident has also been referred to the IPCA.