HMNZS Manawanui: What we know about the Navy ship's sinking

The HMNZS Manawanui capsized and sank on Sunday morning after hitting a reef off the southern coast of Samoa. Photo: Profile Boats / supplied

The Navy ship HMNZS Manawanui has sunk off the coast of Samoa after running aground on a reef.

The 75 passengers and crew were evacuated into life rafts after the incident on Saturday night - a move that authorities say likely prevented loss of life.

But what caused the crash, what's next and how much will the ship cost to replace?

Here's what we know.

The HMNZS Manawanui was on its third deployment to the southwest Pacific this year after leaving Auckland in late September.

The ship is a specialist dive and hydrographic vessel that is used to carry out a range of support, salvage and survey tasks.

Crew on board had been conducting a reef survey before the ship ran aground about 1 nautical mile off the south coast of Upolu, Samoa's most populated island.

Unsuccessful attempts were made to get the ship off the reef before it began to list (lean to one side).

The passengers and crew were evacuated using four life rafts and two rigid hull inflatable boats, Chief of Navy Rear Admiral Garin Golding said.

Samoa Fire and Emergency Services Authority staff help the survivors ashore. Photo: Samoa Fire and Emergency Services Authority

Due to the "extremely challenging" conditions, it took the first survivors five hours to get to shore. An Air Force P-8A Poseidon was deployed to assist.

One of the small boats "flipped on the reef" during the rescue attempt and those who were on board walked to land, Golding said.

Samoa fire commissioner Tanuvasa Petone said three people who were on board needed hospital treatment.

"They ... have some minor injuries, and so we treated them at the site and transferred to them to the hospital."

Golding said one person had a dislocated shoulder, while another had injured their back.

After all the survivors were ashore, the ship caught fire, capsized and sank. It was underwater by 9am on Sunday.

The Defence Force is sending planes to Samoa to bring the crew home. The first was expected to leave New Zealand on Sunday.

About 6pm, people at the Whenuapai air base reported that a Super Hercules was being readied for Samoa. It would be its first international deployment.

The plane tracking website FlightRadar showed the Hercules took off on Sunday evening.

A dive team would be on board to assess salvaging options for the ship, Golding said.

There would also be welfare support on board the plane for the crew.

The Defence Force is also working with local authorities to assess the environmental impacts of the ship hitting the reef.

Defence Minister Judith Collins said the reason for the grounding was unknown and it was too early to speculate.

She said there would be a Court of Inquiry to establish exactly what had happened.

The cost of the salvaging operation and the Court of Inquiry is currently unknown.

However, the HMNZS Manawanui itself will likely cost more than $100 million to replace.

In 2018, the government said the ship would cost $103 million to buy and convert. It was completed in 2019.

Adjusting for inflation, that would be about $129 million today, according to the Reserve Bank.

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