While I think that submarines would be a good part of our deterrent, I would not place them before an ACF. This is for two reasons, the first is that they cannot cover the air domain and the second is that an AFC can get to the aggression point far quicker due to it's quicker coverage of the area. But submarines as a part of the defence system, most certainly add to it. In short what I think we need is a well balanced force.
I disagree. Whilst subs are potentially a very valuable ISR and naval asset, they are very much a specialist capability and an expensive one at that. Given that some of the 'normal' naval roles and capabilities cannot realistically be provided by subs, then I just do not see them having a place in the RNZN or wider NZDF as a whole unless/until there is a major war outbreak. Even then, I believe that NZ would be better off focusing on other capabilities that are either more broadly applicable, easier to acquire (and sustain) or more likely, both.
Consider the following circumstances. The RAN has been a sub operator for nearly sixty years (since ~1967 with the commissioning of HMAS Oxley S57) with a total regular personnel size some seven times that of the RNZN. Despite this, the RAN has noticed, for years, that the current force size of six
Collins-class subs has been a bit of the small side, with the lack of numbers and personnel making it difficult for the RAN to have sufficient numbers to reach a 'critical mass'. Similarly if one were to look at the RCN, they too are sub operators (sort of, anyway, the
Victoria-class subs AFAIK have not had much activity for long periods of time). And whilst having larger populations, numbers of personnel and defence budgets, both Australia and Canada have encountered issues getting their subs into service and at least as important, keeping them in service.
In terms of just raw numbers (personnel and vessels) the entirety of the RNZN would likely need to convert over to operating and sustaining subs in order for the RNZN to reach a four sub navy. Likely at least this many would be needed to ensure that there is at least one sub either available for or already on an op or deployment. Now should the RNZN become entirely a submarine service, that would also mean the RNZN giving up all the capabilities provided by operating surface warships. This in turn would mean no operating NZDF sealift, surface escort, air defence, etc.
To put it another way, if the RNZN was a sub-only force, then the NZDF would have no ability to deploy troops by sea except via STUFT and/or chartered vessels and also no ability to escort other vessels or provide maritime security. A sub is not going to surface to conduct a boarding op on a suspected illegal fishing or smuggling vessel, whilst an OPV or even warship could do so. Same goes for providing a merchantmen an escorting presence if/when it needs to transit threatened waters like off the coast of Yemen.