Mystery surrounds the cause of a fire at Hillmorton Hospital in Christchurch.

A St John's ambulance spokesperson said four people were transported from the mental health services facility to Christchurch hospital.

The four are being treated for smoke inhalation at Christchurch Hospital, a Canterbury DHB spokesperson confirmed.

The fire caused minor damage to the facilities, and nobody was treated for burns.

The spokesperson could not confirm whether those people were staff or patients.

St John's says four people have been transported to Christchurch hospital in a moderate condition.

It is still not known what caused the fire.

Fire and Emergency are not making any comment on the fire.