Rescue officials searched the South Pacific's waters for 33 people missing after a ferry sank during the night off the coast of Tonga, New Zealand rescue coordinators said on Thursday.

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

He said the ferry was carrying 45 passengers and 30 crew members when it went down about 86 kilometres northeast of the Tongan capital, Nuku'alofa.

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.

Three vessels were in the area searching for survivors, and a fourth rescue vessel was on its way there, Blackmore added.

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 pm Within minutes, the rescue centre 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 (25 Celsius)

"There's a lot better chance of surviving in that sort of temperature," Blackmore said.