Wellington - Search teams looking for the Tongan ferry Princess Ashika say it may lie out of immediate reach on a shelf more than 100 metres below the surface.

Authorities estimate that 93 bodies remain trapped in the vessel that sunk Wednesday evening around midnight.

Dive team chief Lieutenant Commander Andrew McMillan told German Press Agency Tuesday that crews had spotted a rope on the water that could be attached to the ferry.

But the Australian and New Zealand dive teams' equipment would only allow them to explore depths up to 100 metres, McMillan said.

He also noted that bad weather was hampering search efforts.

Tonga's transport minister Paul Karalus resigned early Tuesday evening in Tonga. He told Radio New Zealand that he stepped down resigned because he did not want to appear to prejudice any investigation into incident.

Karalus refused to accept any responsibility for the sinking. He told the radio station that "the vessel was seaworthy at the time the survey was undertaken."

The minister's resignation followed the ferry captain's claims to Tongan media that the government knew the vessel was not seaworthy, and ignored the captain's requests for repairs.

The long-serving politician and People's Representative Akilisi Pohiva also told dpa that the sinking of the ferry was "manslaughter by negligence."

Pohiva said he was one of many people who had warned the government that the Princess Ishika was not safe.

Police commander Chris Kelly said authorities were still trying to establish the identity and number of passengers on board.

He told a press conference in Tonga on Tuesday that two foreign nationals believed to be passengers had contacted authorities and said they were not on the ship.

Kelly also identified the second body that police had recovered. She was a 22-year-old Vaefetu'u Mahe, whose husband and child are among the 93 passengers who remain unaccounted for.

Kelly said he hoped to release a provisional list of passengers on Wednesday.

Authorities had previously said the ship carried one Japanese crew member, and two French and and two German passengers. The body of a British national Dan MacMillan has been recovered.

Kelly said it was possible that search teams authorities might never find the ferry.

The number of missing passengers is almost three times the initial estimate of 33 missing people.

Most of those are suspected to be women and children who were asleep when the boat sank around midnight off the coast of Tonga.

About 149 people were on board the ferry when it sank.

Police operation head Sokopeti K To'ia issued a statement Monday morning which confirmed that the known survivors are 54 Tongan males, aged between 10 and 70 years old.

Authorities said they suspect a wave hit the vessel and threw several vehicles and containers held in cargo to one side of the boat, causing it to capsize.

Water quickly poured through the low windows of the ferry, causing it to rapidly sink. (dpa)