If NZ was a larger nation I would agree with you, but it just isn't. Major & medium powers can afford the duplication of effort of a navy and a coastguard, small nations like NZ can't. It's better to just have one agency do both military & constabulary roles. In a small force like the NZDF, still operating in a low threat environment regionally, an increased constabulary role is a fact of life. It's a question of balance, not focus*. Our own white papers highlight the 'concentric circles' approach of local NZ roles being the highest priority**. If our navy doesn't do constabulary work, it will become even more irrelevant. Perhaps constabulary work could become a separate command (as is the case in Norway), but it should remain part of one organisation.
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There are also very strong legal efficacy reasons to do with UNCLOS, IHACL and territorial law enforcement that have clear distinctions between what is a military naval role and a civilian law enforcement agency operating with a nations territory. A number of countries are looking to or have established Maritime Enforcement Agencies that separate military maritime forces from law enforcement. Indonesia and Malaysia for example. The other Scando nations of a similar size also have a separate Coastguard / Maritime Enforcement agency to their Navies. They too are of similar size to NZ. So the big power analogy you are proposing is not always applicable on those grounds. It is also not a true duplication of resources because there are clear demarcations in operational rationale, law and resources.
I have not read the Norwegian Coast Guard Act of 1997 but I assume that it does have clauses that deal with issues of legal efficacy and its subsuming into the Norwegian Defence Act. Their Coastguard Act was a way of demarcing the very legal issues that we have under our present administratively convoluted arrangements. In my view their coastguard command as part of the Navy is better than what we do. Nethertheless, I am very much against the blurring of what is law enforcement and what is military. The Navy as it is - is too small for a diversity of vessels ranging from small inshore craft (0-24nm) that are needed for law enforcement through to our future globally orientated Anzac replacements and large support vessels.
There are at least six NZG civilian agencies attempting to work with the NZDF in the maritime space - most of it focused within 24nm CZ, most of law enforcement related but more will develop in the environment protection sphere.The current one NZ size fits all approach of a 'maritime law enforcement' role and a 'military' role is failing both government goals and is simply schizophrenic. Also in the one area where more needs to be done 'maritime environmental protection' there is nothing happening other than piddling around the edges even post Rena. Will that focus be added to the Navy as well?
In a jurisdictional application sense it is like combining the Army (military) with the Police (civilian law enforcement) and this was the legal difficulty surrounding nations and their role in the littorals. Our current approach does nothing but degrade both roles (military and law) enforcement operationally, legally and fiscally.
The Navy will not become irrelevant if it focuses on its traditional military role. New Zeland will come under more pressure from more than what happens in within its 24nm CZ in the decades ahead. We do not live in a benign strategic environment and we need to focus on addressing that. We can start by not diluting the NZDF with law enforcement tasks within the CZ.
I also agree with the Admiral in the link to the RAN Sea Power Conference. Particularly with respect to balance. There is a role for the RNZN within the constabulary aspect of the wider NZ EEZ and her interests but the focus should be balanced towards on the outer EEZ and not within the CZ unless bought into support the lead maritime law enforcement agency.
The concentric circles does not entirely mean the total responsibility lies in the defence of the realm with the NZDF. There are also NZSIS, GCSB and hopefully a Border Enforcement Agency established who are all part of that concentric model you noted.