Nor is intelligence-sharing between New Zealand and Japan non-existent, according to former intelligence and defence policy analyst Dr Paul Buchanan.
"They already do [share information], via Five Eyes and bilaterally, say on Yakuza involvement in drugs or people smuggling. But full incorporation into Five Eyes gets into sensitive technology, and strategic as well as tactical targeting data transfers, which at present is not the case."
So while enhancing the intelligence relationship with New Zealand may not seem like a big step up from the status quo, it's the logical next step in testing the waters for Japan turning Five Eyes into Six - which would be a big step up.
One reason cited for that not already happening is that Japan's intelligence agencies are still perfecting their information protection measures.
Hence the wording of the New Zealand-Japan agreement: to set up a legal framework so sensitive intelligence can flow seamlessly between countries in a protected way.
The timing also seems very deliberate, as well as the framing. In Ardern's words, it will "support peace, stability and security in the Pacific and the wider Indo-Pacific region".
The China-Solomons security pact was signed this week, which has led to escalating concerns about the possibility of Chinese military presence - and even a base, close to strategic ports - in the Pacific.
China, in essence, wants to make sure it can step in with whatever means necessary to protect its interests. Such capability in the Pacific would go a long way to safeguarding the billions of dollars China has spent there.
New Zealand opposes anything that could lead to the militarisation of the Pacific, but it's a minnow against China. It needs hefty helpers.
Japan has heft. It's the world's third largest economy. It shares New Zealand's support for a rules-based global order which the Russian invasion of Ukraine, in Kishida's translated words, has "shaken the very foundations of the international order."
The announcement also has symbolic power. China will hardly be overjoyed at New Zealand and Japan more closely joining spying forces.
China - and Russia - will also notice if it means the Five Eyes door to Japan becomes more open. Japan is much closer in proximity to both of them, as well as North Korea, compared to Five Eyes countries.
Japan already cooperates with Five Eyes on an ad hoc basis, but Buchanan says formally joining would be a significant change.
"Perhaps the People's Republic of China's behaviour has something to do with that."
It's easy to see this as a major coup for Ardern, who before last night had never met let alone spoken with Kishida.
The reality, though, is that such agreements aren't done overnight, but are a long time in the making before getting to the stage of Prime Ministers' sign-off.
Ardern will nevertheless be very happy with the joint statement that was released last night.
She came to Japan seeking closer ties to friends in a world where peace looks increasingly threatened, and with no established rapport with the new Prime Minister.
She will leave tomorrow with such rapport, as well as an agreement full of exactly the kinds of closer ties she was seeking.
