A Navy ship came close to capsizing with 75 people aboard in treacherous conditions in the Southern Ocean, Defence Minister Ron Mark has revealed.

The potential for loss of life was high but the HMNZS Otago's commanding officer managed to manoeuvre the vessel through mountainous seas and floating ice.

Mark said the incident took place in huge waves at least 20m high which were a result of climate change and which would affect the types of ships New Zealand needed in the future.

Mark said he was told of the incident by Lieutenant Commander Lorna Gray, commanding officer of the offshore patrol vessel Otago, after she and her crew returned from a nine-day deployment to the Southern Ocean last December to seek out illegal fishers.

It was Gray's first mission to the Southern Ocean since taking the helm of the Otago in June 2017.

Mark recounted the story today during the launch of the Strategic Defence Policy Statement which has highlighted climate change as a challenge for the Defence Force.

"The sea state [Gray] described to me, the conditions they're now encountering with ice that they cannot safely navigate through," Mark said.

"The manner in which she had to manoeuvre HMNZS Otago to prevent it capsizing, and the potential for her losing her complement was high, very high."

There were about 75 people on board including Royal New Zealand Navy crew and staff from the Ministry for Primary Industries and GNS Science.

A Defence Force spokesman said: "The Royal New Zealand Navy always expects testing condition in the Southern Ocean and is confident that its personnel are trained to deal with any maritime conditions."

"We are witnessing right now, increasing frequencies of increasingly violent, disruptive and destructive weather patterns.

"Ice-strengthening, ice-capable vessels have got to be on the radar. If we add to that the military capability, we're starting to get a picture of what we need," Mark said.

"Global warming is real. It's happening, the consequences are being seen and our defence force people are in the front line."

The Defence document is also unusually explicitly critical of China for a New Zealand policy statement.

The paper says that as China had integrated into the international order, it had not consistently adopted the governance and values championed by the order's traditional leaders.

"Both domestically and as a basis for international engagement, China holds views on human rights and freedom of information that stand in contrast to those that prevail in New Zealand."

It says China has set an alternative model of development - a liberalising economy without liberal democracy – challenging conventional wisdom in the West that the two go hand-in-hand.

Ron Mark equated its frankness on China to its nuclear-free position and being the first country to give women the vote, rather than it being a "pick-on-China moment".

"We should never shy from being frank and honest because that's not the way New Zealanders have done it in the past.

"New Zealand is an independent nation; it has an independent foreign policy. Sometimes we need to be prepared to make ourselves heard and to back ourselves."