A Royal NZ Navy ship which ran aground and caught fire has since capsized off the coast of the Samoan island of Upolu, authorities have confirmed.

The HMNZS Manawanui "was known to have capsized and was below the surface" by 9am, the NZ Defence Force confirmed this afternoon.

The vessel was on fire and listing heavily, after running aground before 7pm last night. Photos from the scene showed smoke billowing from the stranded ship.

All 75 crew and passengers onboard were safely rescued overnight. Two people needed hospital treatment for minor injuries, one with a dislocated shoulder and another with a hurt back.

Defence Minister Judith Collins told media in Auckland there will be a Court of Inquiry launched to establish why the ship ran aground, caught fire and then capsized.

"The immediate focus right now is on mitigating the environmental impacts and salvaging what is left of the Manawanui."

The ship ran aground near the southern coast of Upolu, with all on board evacuating to "life rafts and sea boats", according to the Defence Force.

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"Rescuers battled currents and winds that were pushing the life rafts and sea boats toward the reefs, and swells made the rescue effort particularly challenging.

"Those on board the lifeboats and sea boats were transferred to vessels that responded to the rescue call and were transported ashore."

The Defence Force said "the exact cause of the grounding is unknown and this will need further investigation".

"The NZDF is working with authorities to understand the implications and minimise the environmental impacts."

A witness nearby told 1News they saw flares being fired into the sky at about 9pm last night and saw the "vessel tilting on one side". The ship had been carrying out a reef survey when it ran aground.

An Air Force P-8A Poseidon was later deployed to the incident.

Maritime component commander commodore Shane Arndell said the people rescued from the ship were "being supported in Samoa or on supporting vessels".

"The safety of personnel is our top priority," he said.

"The New Zealand Defence Force is working closely with the Rescue Coordination Centre (RCCNZ) who are coordinating rescue efforts. Several vessels responded and assisted in rescuing crew and passengers."

The Defence Force said 78 people were onboard the ship in a statement just after midnight, but later revised it to 75 in an update.

The ship, which was in Samoa ahead of the Commonwealth Heads of Government meeting later this month, was stranded off the Sinalei Reef Resort and Spa where King Charles will be staying.

The incident occurred while the vessel was conducting a hydrographic survey, one nautical mile from shore. The 75 crew and passengers from other government agencies began evacuating into lifeboats at 7.52pm, according to the Defence Force.

Local man Will Iopu, who was at the hotel, told 1News he had headed to the beach around 9pm for a barbecue when he saw people looking out to the sea towards the ship. Police arrived at the scene shortly afterwards.

"About 9.20pm I could see the first flare go up in the air and that’s when we knew things were a bit serious," he said.

"After the first flare, three minutes later a second flare went off."

Iopu said Sinalei hotel staff took one of their boats out to see if they could help. They were joined by staff from nearby hotels with boats in tow.

By 11.30pm, he said he could "see the lights on the vessel tilting on one side".

On X, Defence Minister Judith Collins said: "My thoughts this morning are with the officers, crew, and passengers on the HMNZS Manawanui.

"Many thanks to the Samoan people for their brave assistance. Our families will be shocked at what happened, but grateful for the successful rescue."

Labour's defence spokesperson Peeni Henare said the incident was "devastating".

"My first thoughts are with the safety of the crew onboard the HMNZS Manawanui, and I'm relieved to hear they're all safe and out of harm's way. I certainly want to recognise the incredible efforts of those who helped coordinate and rescue our New Zealand crew.

"I'm confident New Zealand and fellow partners will be working around the clock to respond appropriately and support Samoa during this tough time."

This morning, the Samoa Fire and Emergency Services Authority shared photos from the scene on Facebook, saying it was fortunate "no one was heavily injured".

"We're proud to say WE SAVED THEM," the authority added.

"If you're unsure about our sea areas, please BE CAREFUL, TAKE CAUTION, and STAY SAFE!"

The HMNZS Manawanui was the navy's specialist dive and hydrographic vessel able to conduct specialist missions including explosives disposal, salvaging, surveying, and underwater search and recovery.

The Manawanui was in Samoa in May conducting surveys of the Mulifanua Channel and Apia Harbour to update charts last done in 1987.

In June and July, the vessel worked alongside the Australian Navy disposing of WW2 bombs and surveying areas of the sea floor in Ha’apai, in central Tonga.

The HMNZS Manawanui was purchased for $103 million by the Defence Ministry in 2018.

Then-defence minister Ron Mark said in a press release at the time the ship would “fill an existing capability gap” created when two vessels were decommissioned – the survey ship HMNZS Resolution in 2012 and dive tender HMNZS Manawanui in 2018.

Norwegian research vessel Edda Fonn was retrofitted with the dive and hydrographic systems required by the Defence Force.

Onboard included a 100-tonne salvage crane, a remotely operated vehicle and a contemporary dynamic positioning system which would allow “greater levels of effectiveness and safety” for Navy divers.

Mark said the Edda Fonn was subjected to “considerable scrutiny ahead of purchase”.

“We have been assured by independent experts that it is in excellent condition, and will handle well in the operations the Defence Force will use it for.”