Defence Minister Ron Mark announces the purchase of four Boeing P-8s to replace the ageing Orion aircraft.

The Green Party says New Zealand is not truly subscribing to the rules-based order by purchasing new military planes with increased combat capability.

On Monday, Defence Minister Ron Mark announced the Government would buy four Boeing P-8A Poseidon maritime patrol aircraft from the US Government at a cost of $2.3 billion.

The planes, which have enhanced weapon and submarine-hunting capabilities, will be delivered from 2023, with the cost spread out to 2026, and would replace the fleet of ageing P-3K Orions.

Mark said the P-8s were the only choice, and would strengthen the Government's Pacific reset.

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Meanwhile, new New Zealand Defence Force (NZDF) chief Air Marshal Kevin Short said the Government wanted the option of the full range of military capabilities - "unless you buy that upfront, you never have that option".

"We are a military force, and what we wanted for the Government is a response option," Short said.

Mark and Foreign Minister Winston Peters have spoken extensively about the growing threat to the global rules-based order. And it seems part of New Zealand's response was to ensure the NZDF is combat capable, and has interoperability with its traditional allies.

RICHARD MYBURGH/SUPPLIED Green MP Golriz Ghahraman says the Government has been talking ab out the importance of the global rules-based order, but it should practise what it preaches and stop investing in weapons proliferation.

But Green Party defence spokesperson Golriz Ghahraman said New Zealand needed to move away from investing in war industries if it was serious about upholding the international rules-based system.

"We need to move away from following the old world order of the US, UK, Australia and diverse from investing in the war industries and focus on what we can do best, instead," Ghahraman said.

New Zealand should focus on search and rescue, and supporting scientific research, she added.

​"I think we are really well-placed to sing from that songsheet."

But there was pressure in the international community where New Zealand had "constructed ally ships" to continue to invest in combat capable equipment, like the P-8s.

"New Zealand has always been really great at building our own path, especially when it comes to peace and disarmament... My hope is we do stand up for a different approach."

Earlier this year, the Government revived the ministerial portfolio of disarmament and arms control, which Peters had been appointed to.

"The point about the global rules-based system is that the rules, and adherence to those rules are dependant on nation states, like us," Ghahraman said.

"Our practice of investing in the war industry is part of eroding the move towards non-proliferation of arms, of not relying on war and the use of force to solve disputes.

"If we want a commitment to the rules-based system, we have to practise what we preach. And part of that is divesting from the war industry."

Mark and Short said there was no pressure from Australia to purchase the P-8s, but there were discussions.

Australia, and the US, which both used the P-8s, would have confidence in New Zealand's purchase.

Mark said he was "a P-8 sceptic", but the Government believed the planes were the best option available to replace the P-3s.