Up to 117 people are now believed to have been on board the Princess Ashika when it sunk late on Wednesday night.
Rescue Coordination Centre New Zealand mission coordinator John Dixon said there are still 62 people unaccounted for and two confirmed dead.
"There is still a significant amount of debris in the water, and we will be targeting those areas.
"Clearly survival rates after this length of time are of concern, but we remain hopeful of finding more survivors," Mr Dixon said.
A survivor said the vessel rolled so quickly in rough seas that women and children sleeping below decks had no chance to escape.
The crewman said the captain of the Princess Ashika had warned of rough seas approaching with less than a minute's notice, said Daniel Tuita, operations officer for the Tongan Navy.
"A very short minute after the captain warned them, they tried to yell out to the people inside, but the accident happened so suddenly."
All the women and children were inside, sleeping and taking refuge from the rough seas, while the men were outside smoking and lying on the deck.
"So when the accident happened, unfortunately those who were safe inside were the first to go down," Mr Tuita quoted the crewman as saying.
The first rescue ferry to arrive was the Pulupaki, and two container ships were also called to search for survivors.
Siaosi Lavaka told the Matangi Tonga news website that the fellow survivors he saw packed aboard life boats were all men.
"No women or children made it," Mr Lavaka said after being returned to shore with about 50 other men.
Ships and planes, including two Royal New Zealand Air Force Orions, searched waters near Tonga yesterday, while the body of a Scottish man, believed to be Daniel MacMillan of Christchurch, was recovered.
Mr MacMillan who had been on holiday in Tonga, was initially identified as a New Zealander because a NZ driver's licence was in his pocket when his body was found.
Officials said 55 survivors had been plucked from the ocean after the Princess Ashika overturned around midnight on Wednesday 85km northeast of the capital, Nuku'alofa.
Mr Lavaka, whose mother is among the missing, said he woke to find the ferry rocking violently and waves breaking over the lower deck.
The rocking apparently moved cargo to one side of the ferry, unbalancing it and turning it over.
"We woke up to the sound of shouting and we jumped off," he told the Matangi Tonga website.
There were nine lifeboats and seven were all filled with men.
Tongan Prime Minister Fred Sevele said last night he believed most of those on the boat were Tongan. There were two Europeans - including the British man carrying a New Zealand drivers' license whose body had been recovered. Another was believed to be Japanese.
He said he had asked Australia and New Zealand to provide some Navy divers to help recover any bodies trapped in the ship. Most of whom were believed to be women and children sleeping in berths below decks.
"We do believe the missing ones are down with the ship so it is important that we get some people to see if we can recover them."
He said hopes for those missing were fading.
"The hopes for the rest are not promising, unfortunately. It is now over 24 hours and in all probability the chances are not that great."
He said he would meet with the families of those lost when he returned home from Tonga.
"It's very sad for a small place. This is a huge disaster and a sad loss. We will try to cope with it as best we can."
Briton Emily Penn - who works with the Kiwi-based charity Sustainable Coastline at Pangai on Lifuka Island - went down to the wharf with locals to help out when the rescue vessels arrived.
When Mr MacMillan's wallet was discovered - bearing his identification details written in English - she offered to translate.
Ms Penn spoke to English-speaking survivor Pau Tupou. "He said it was about 11pm when the waves started crashing into the side of the boat and it started to tip.
"It sounded as if the cargo moved over to one side of the boat."
It all happened extremely quickly.
"He said within a minute the boat had capsized and all of the women and children were sleeping inside."
Mr Tupou said all those who could, swam to the life rafts and waited for the first rescue vessel, which arrived in about 25 minutes.
Ms Penn was told the Pulupaki had taken on board dozens of men. Some were dropped off on the island of Ha'efeva on the way back to Pangai.
Ms Penn understood the Princess Ashika's captain had been flown to the main island of Tonga Tapu to be treated for severe shock.
Rescue vessel captain Sioape Fainga'anuku said staff carried on searching for a while, but then decided to suspend the hunt until daybreak.
Captain Fainga'anuku told Ms Penn it was impossible to know how many were on board the Princess Ashika as nothing was ever documented and there were usually about 10 stowaways on each trip.
Auckland University of Technology tourism professor Simon Milne, who is in Tonga, told NZPA the ferry was carrying two ambulances, which were to be delivered to hospitals on the islands.
Survivors were last night being interviewed by police to establish how the ship went down.
A list of the rescued 27 crew members and 23 passengers was issued yesterday afternoon.
John Jonnesse, managing director of the Shipping Corporation of Polynesia, said the 50 people were being kept separately to ensure there was no collaboration on stories.
He said each person was being interviewed by two police officers and he anticipated the process would take until late last night.
"This is so that each person gives their own version of events," he told the Herald.
"What police will do now is correlate those accounts and put it into a report and that will go back to the police commander in Tonga."
Mr Jonnesse hoped to talk to the survivors to ensure they had been put in touch with families.
He said most of the people were being put up in private homes in Ha'apai overnight.
Those on the ferry who were due to continue to the island of Vavau would be flying there today, he said.
Taliofa Koloteaua, crewing manager for the corporation, said the Princess Ashika had arrived in Tonga in July. It had previously been used as a tourist boat in Fiji.
Ms Koloteaua said the ferry was equipped with more than 300 lifejackets - more than enough for crew and passengers. There were also liferafts on board.
A mayday call was received about 11pm on Wednesday and New Zealand's Rescue Co-ordination Centre launched a major search.
An RNZAF Orion arrived at first light, searching about half of the 207sq km search area before it was replaced by another Orion that left New Zealand at 8.30am.
Flight Lieutenant Nathan McMaster said a trail of debris stretched about 15km from the spot the ferry is thought to have plunged into the Pacific Ocean.
The ferry's cargo was scattered among the debris and liferafts, some partly deflated, floated empty.
Flight Lieutenant McMaster said the first boats to reach the area rescued 53 people and took them to Ha'feva.
Meanwhile, Prime Minister John Key yesterday personally expressed New Zealand's sympathy to Prime Minister Sevele in Cairns, where they are both attending the Pacific Islands Forum.
"Our condolences and our thoughts go the people of Tonga," Mr Key said.
"We're very distressed to hear about the collapse of the ferry and the loss of life of up to 30 people.
"This comes on the back of the loss of another ferry a few weeks ago in Kiribati. I think it's a sign of the fact that there are real issues actually with the quality of some of the ships that are operating in the Pacific."
Foreign Minister Murray McCully said in a statement: "We are deeply concerned about those still missing."
The Princess Ashika, built in Japan in 1970, had been plying Tongan waters for only a few weeks and was just a stop-gap measure ahead of a new ferry coming into service.
The Princess Ashika was shifted from Fiji to replace the old 'Olovaha because of fears that that passenger ferry was no longer safe.
Prime Minister Sevele confirmed the boat was carrying cargo, including vehicles. While the cause of the sinking remained unclear, he said one theory was that the vehicles were not properly stabilised and had moved about.
He understood the boat was at a depth of about 35 metres.
He said the boat had operated in Fiji for about 30 years before being sent to Tonga, but it was tested for seaworthiness and had passed stringent Lloyds' insurance tests. Tongan crew had tested the boat in Fiji for several weeks before it was taken to Tonga.
"We were assured it was seaworthy and was tested and passed the test. We were quite satisfied according to reports we got before we actually paid for the ship."
The ship did an important route and he welcomed the offer of
"I'm very happy with the spontaneous response from both Prime Ministers."
He said there had been about two major accidents over the past 50 years - including one sinking about 15 to 20 years ago in which 60 to 65 passengers were lost. Six lost their lives in another boat accident in 1994.
He said there would be an investigation to ensure safety measures were adhered to.
Tongan ferry disaster: 117 on board, 62 missing
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