Rescue officials searched the South Pacific's waters for 27 people missing after a ferry sank during the night off the coast of Tonga, New Zealand rescue coordinators said Thursday.

The ferry sank fast "but we don't know why," said New Zealand Rescue Coordination Center spokesman Neville Blackmore in a statement.

New Zealand is responsible for search and rescue activities in the Tonga region.

Blackmore said the ferry was carrying 49 passengers and 30 crew members when it went down about 54 miles northeast of the Tongan capital, Nuku'alofa. Officials revised the total number of passengers to 79, up from an earlier count of 75.

Blackmore told New Zealand's National Radio that most of the 42 people rescued had been on seven life rafts.

"But we understand there were only eight life rafts ... so we are looking for people in the water," he said.

Another 10 people were later pulled from the ocean by three vessels in the area searching for survivors. A fourth rescue vessel, from the Tongan navy, was on its way to the search area, Blackmore added.

A Royal New Zealand Air Force P3 Orion maritime surveillance airplane was also searching about 80 square miles of ocean for survivors, he said.

The Princess Ashika ferry was heading from Nuku'alofa to Ha'afeva, in the nearby Nomuka Islands, when it issues a mayday call about 11 p.m. local time. Within minutes, the rescue center received a distress beacon alert from the vessel, Blackmore said.

There were strong winds at the time, but the water's temperatures were about 77 degrees."There's a lot better chance of surviving in that sort of temperature," Blackmore said.

The ferry, built in about 1970, was being used as a fill-in vessel until a new ferry was completed, Blackmore said.