Yes exactly.
One Collins Class submarine operating in the Tasman Sea completely swings the odds against Communist Chinese Task Group 107.
The PLAN has a glaring weakness in ASW and operating in our neighborhood at the periphery of their empire they could not bring the air assets in that are required to prosecute a New Zealand sub surface threat.
Just the existence of New Zealand submarines completely changes their required force composition and massively complicates their plans, creating shadows of doubt and concern in the minds of our Communist enemies.
Not really. Australia has struggled to manage and sustain a force of six subs, with a significantly larger defence budget as well as potential pool of personnel to draw from. In order for the RNZN to manage to have a single sub either available for or on a deployment somewhere, then NZ would likely need a minimum of three and IMO more likely four subs in order to manage that. The RNZN would also likely have problems managing to sustain an adequate number of personnel to actually crew that few boats (a problem the RAN has encountered trying to sustain ops for six boats).
If essentially all the NZDF resources were directed to subs, then NZ might be able to manage it, but such a force structure distortion would play havoc with everything else that the NZDF needs or might need to do.
Subs are great ISR and anti-shipping assets and depending on design and armament configuration can also be great platforms to launch strikes. However they are useless at protecting surface shipping from aerial threats and likely would not even be aware of when merchant shipping might be threatened by many of the potential dangers to shipping. They also cannot replicate any of the capabilities provided by elements of the RNZAF or NZ Army.
If (big IF) Vote Defence does get sustained funding increases in real terms, there are a number of areas where more and sustained funding could be put to use. Unfort establishing a Kiwi sub force is not such an area because there are too many other areas which would need coverage first.