ANALYSIS: The AUKUS partnership has New Zealand’s only formal defence ally, Australia, turning nuclear-powered. And it has highlighted a sizeable difference in military might.

Australia, with the AUKUS defence pact announced on Thursday, has thrown its chips in with the United States and United Kingdom, promising to acquire at least six nuclear-powered submarines and pile up missiles in the face of an increasingly strident China.

New Zealand was never a realistic contender for such a nuclear-powered pact, given its longstanding anti-nuclear stance. And countries and militaries of New Zealand’s size simply can’t play in the submarine leagues; acquiring such a vessel for the naval fleet was deemed “fanciful” when considered in the 1980s.

However, experts in the New Zealand’s military capabilities say the AUKUS pact will put the pressure on the New Zealand Government to commit to an important military asset: new naval frigates.

"It's certain that Australia, at least, will be saying, 'Well you're a military ally of ours, what are you gonna do?'” former defence minister Wayne Mapp said

“When you are in a military alliance, it has obligations as well as advantages. There's no bucking that fact, and we can't hide behind the nuclear-free thing and say, 'Oh that answers everything'. It doesn't.”

Australia already has a fleet of six diesel-electric submarines, most of which are more than 20 years old, and was planning to replace them with conventionally powered submarines built by France.

But, in tearing up the French contract and choosing nuclear submarines, Victoria University professor of strategic studies Robert Ayson said Australia would gain “formidable fighting machines” that were quiet, could stay underwater “pretty much indefinitely”, and could travel further.

“Australia seems to be aiming more for that projection further north scenario ... This is about deterring, or making it very difficult, for China to met its strategic objectives.”

Imagine a scenario, Ayson said, in which China was trying to get beyond the chain of islands which border its seas, and threaten the US’ naval assets.

“You’re trying to prevent China from breaking out. One way you can try and do that is raise doubts in China’s mind that, if it does send its own submarines and its surface vessels, they don’t know when the attack might come.

“It’s quite possible that sitting there quietly is a pretty potent deliverer of strike.”

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New Zealand was simply unlikely to get involved in such long-range strike capabilities, he said. But purchasing submarines was considered in the 1980s.

"What we decided is that there would be advantages to New Zealand having submarines, but if we pursued submarines that would be the only thing we could have in our defence force, because for us so costly to develop.

"Think of how much it would cost to train and prepare submariners. We're no longer doing that for fighter pilots.”

Instead, Australia’s commitment would likely mean New Zealand returns to a debate about whether it needs to replace its two frigates, HMNZS Te Mana and Te Kaha of the Anzac class.

"Apart from those, and the P-8s [maritime patrol airplanes], there’s not an awful lot which we can throw into these East Asian maritime conflict zones.”

Mapp, who was defence minister from 2008 to 2011, said defence assessments in the past decade made clear the frigates were one of the three “critical” pieces of military hardware that needed upgrading.

“This particular [AUKUS] announcement will put quite a bit of pressure on the New Zealand Government to make it clear how they're going to replace the Anzac frigates, because they can't wish that decision away.”

The prior Labour-coalition Government had committed to replacements for the Orion surveillance aircraft with four P-8 Poseidons at a cost of nearly $2.5 billion, and replacing the ageing C130 Hercules with five new planes of the same model for $1.5b.

The frigates are undergoing upgrades that would extend their use into the 2030s. But, Mapp said, it could take a decade to replace the vessels.

“We all knew that the top one of those three would in fact be the frigates. We always knew the frigate question would be the tough question.”

The Australians are buying nine new “Hunter class” frigates to replace its fleet of eight Anzac class frigates, which were jointly acquired with New Zealand’s current frigates.

The United Kingdom and Canada were also acquiring the same type of warships for their navies and, Mapp said, New Zealand could do the same.

“There’s a hell of a lot of logic to that, isn’t there?”

He said the current Labour Government, which has a majority in Parliament and is currently re-evaluating the goverment’s defence capability plan, might be advised to commit to the frigates now, as such a decision may be more difficult in any future coalition government.

The Green Party, a possible future coalition partner, has made clear it has reservations about replacing the frigates.

As for submarines, that was simply “a league we can’t play in”.

“There was discussion 40 years ago about it, but it was seen as fanciful and never seriously progressed,” Mapp said.