I think that treasury got in the way. Also we must remember that John Key, PM at the time that the requirement was drawn up was adverse to capital spending. Treasuries excuse was that 4 new P8's could do the same as 6 old P3's. the question also then is what happens when the P8's get older? My personal view is that we need enough to keep our half of the Tasman sea secure for marine transport as a minimum.
So (ideally) we perhaps need another 4 (8 in total) as a minimum, assuming 6 still needed for 1 on station 24/7, leaving 2 for attrition/training/ maintenance cycles? Treasury's world view in the 2010's will be brought up to date with the strategic outlook of the 2020's/2030's etc.
UK bought 9 (which seems to be on the light side as well), be curious as to what the rationale was numbers wise. Perhaps that could guide NZ's fleet outcome?
Our P-8's cost approx NZ$400m ea, so potentially affordable if future funding/prioritisation allows (including increased operational and support funding). Of course allowance for air and ground crews also required (although RNZAF suffers the least of the 3 services when it comes to retention issues and is mostly manageable and much is made of OEM support).
Granted long range maritime UAV's would be supplementary. But they can't keep track of or prosecute underwater objects of interest. With the proliferation of submarines in the Pacific (there are approx 130 CCP/NK subs in the Pacific, assuming the figures are accurate), let alone Russian, which is much greater than the Soviet Pacific fleet sub numbers during the Cold War when the requirement was for 6x P-3's!
There are currently 42 countries in the world that have submarines as a part of their naval fleet. These countries range from superpowers like the United States and China, to smaller nations like Cuba and Myanmar. While submarines are used for a variety of purposes, they are primarily used for...
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These sub numbers also suggest RNZN still needs highly capable future ASW Frigate replacements, rather than a "tier 2" compromise to reduce the number of hull types?