Tonga's Prime Minister Feleti Sevele has asked New Zealand and Australia to send navy divers to help recover bodies from the sunken ferry.

Sevele said it is believed those who are missing went down with the ship, which has been located at a depth of some 35 meters.

Sevele added that the cause of the sinking remains unknown, but there were no concerns over the vessel's seaworthiness, as it had passed safety inspections and was found to be suitable for Lloyd's insurance.