A New Zealand navy ship has sunk in the South Pacific Ocean after catching fire on Saturday night, sparking an urgent rescue operation.

Seventy-five crew and passengers on board HMNZS Manawanui are safe, but three required hospital treatment after the vessel ran aground off the coast of Samoa.

The New Zealand Defence Force said the cause of the grounding is not yet known and would need further investigation.

The Defence Force said on Sunday the navy's specialist dive and hydrographic vessel rang aground near the southern coast of Samoa's main island of Upolu on Saturday night as its crew were conducting a survey of the island nation. 

The over 7,500-tonne, 84-metre-long vessel, after catching fire, later capsized and was below the surface by 9am local time on Sunday, the Defence Force said.

“They’re all on land. They are safe and sound apart from just a few individuals that… have some minor injuries, and so we treated them at the site and transferred to them to the hospital," NZ Radio reported.

The New Zealand Defence Froce said it was “working with authorities to understand the implications and minimise the environmental impacts”.

Chief of Navy Rear Admiral Garin Golding told a press conference in Aukland that a plane would leave for Samoa on Sunday to bring the rescued crew and passengers back to New Zealand.

He said some of those rescued suffered minor injuries, including from walking across a reef. 

Defence Minister Judith Collins described the grounding as "really challenging" for all on board, adding that it will "take quite a bit of time to process" what had happened.

"I look forward to pinpointing the cause so that we can learn from it and avoid a repeat,” she said. She added that the immediate focus was to salvage "what is left" of the vessel.

Videos and photos published by local media showed the Manawanui, which cost the New Zealand government 103 million New Zealand Dollars in 2018, around £48 million, listing heavily with thick plumes of grey smoke rising from it. 

The New Zealand Herald later reported that the vessel had sunk, citing a New Zealander on the shore. This was not immediately confirmed by the Defence Force. 

Rescue operations were coordinated by Samoan emergency services and Australian Defence Force personnel with the assistance of the New Zealand rescue centre, according to a statement from Samoa Police, Prison and Corrections, posted on Facebook.

“Our fire rescue team responded to the Manawanui vessel Navy crew in distress, working from last night until this morning,” the rescuers said. “Fortunately, no one was heavily injured and no lives were lost. We’re proud to say we saved them."

A Royal New Zealand Air Force P-8A Poseidon was also deployed to assist the rescue, according to VOA News.

The vessel has conducted a range of specialist diving, salvage and survey tasks around New Zealand and across the South West Pacific. This has included underwater explosive disposal and underwater search and recovery.

“If you’re unsure about our sea areas, please be careful, take caution, and stay safe,” the Samoa rescuers posted on Facebook.

The country's navy is already working to reduce capacity with three of its nine ships sat idle due to a personnel shortage.