Honestly I think the NZDF would be better served ordering one of the existing LPD designs with suitable modifications, as opposed to ordering an LHD, either one of the existing designs or having a new one custom-designed for NZ.
While the notion of more flight deck and/or hangar space might seem appealing, how much 'more' is really useful? Using the HMNZS Canterbury (an LHA) as an example, there is hangar space for four helicopters, and landing spots for two medium helicopters or one Chinook IIRC. Given that across the whole of the NZDF there are only eight transport helicopters in total, trying to pack more onto a single ship does not sound like a good idea to me.
The advantages of a LHD over a LPD is the LHD having a through deck flight deck enables great versatility of the flight deck and removes significant airflow problems from the superstructure that a LPD can present.
Another area of concern is what would be a reasonable troop and vehicle capacity? Having gone through the various LHD classes currently in service, the smallest (which is really more of a modified LPH) is ~19,000 tons at full load and can lift ~720 troops and ~200 vehicles, while other designs can very the numbers of troops and vehicles they are still significantly greater than NZ would be able to field.
Given the size of the NZ Army, I would consider a troop lift requirement of a reinforced company reasonable, with perhaps some spare capacity in the event of an emergency to lift/move some extra personnel. The whole uniformed portion of the NZ Army (regular and reserves) is only ~6,400 personnel, so it would seem unwise and/or wasteful of resources to purchase a vessel to move 10%+ of the Army in one go, or get such a vessel with the expectation that the NZDF would leave it half (or more) empty virtually all the time.
Whilst the Canterbury, which in reality is a goat boat not fit for purpose (I do not mean any disrespect to Canterbury's crew past or present), was designed to carry a reinforced rifle company, it appears that many people think that, for ever and a day, NZDF will only ever deploy a reinforced rifle company by amphibious ship., which is muddy thinking with little logic, because history has shown that in geopolitics, situations can and will change quickly and not always for the better. So pollies do get a wake up call which causes them great consternation with the last one of such magnitude being in the second half of 1941, resulting in significant grinding of teeth and tearing of hair, in Wellington and Canberra. The end result in late 1945 was that Canberra took the lesson to heart and Wellington didn't, reverting to the prewar 1930s attitude which still prevails today.
There are LHD that will lift < 500 troops, ~ 100 - 150 vehicles, hangar 6 - 8 helos with spots for 4 - 5 helos on the flight deck.
This is an illustration of a
ST E170 Endurance Class LHD that I have done.
You will find that some LHDs designs are scalable and fresh air is free and steel is cheap, so it is good to future proof as well. We don't need to take 8 x NH 90s on a deployment, but 3 - 4 NH90s plus 2 A109s and 2 - 3 SH-2G(A) Seasprites is very doable and that's just us. We'd have no problems filling up a vehicle deck either be it for combat or HADR. By the time you get 30 - 40 NZLAV, trucks, artillery, trailers, engineering vehicles etc., loaded it will start to look crowded.
There are really only two capabilities which I think would be wise to include that NZ would not make use of but would instead be incorporated to provide options to NZ friends/allies. The first is that I would require at least one vehicle deck that is rated and capable of functioning as a heavy vehicle deck, able to handle the weight of MBT's and other large armoured vehicles. NZ has no such vehicles in the inventory at present, and the situation seems likely to remain this way for some time, but being able to land or remove MBT's could enable Kiwi forces to operate with small Australian or US force detachments without also requiring amphibious assets from them. By way of example, if NZ was the lead nation on an international peace keeping/enforcement operation in the S. Pacific or Pac Rim where a small number of heavy direct fire support vehicles were needed or advisable, then it would be less resource intensive if the Kiwi vessel was able to land the personnel, as opposed to requiring each contributing coalition partner to bring their own vessels in to land what might amount of a platoon or less of personnel.
The second would be that the aviation facilities (landing pad and hangar) should be able to handle larger transport aircraft like Chinooks and Ospreys. Again, this would be to enable better inter-operations between NZ and friends/allies.
Historically we have always been an expeditionary force (apart from the NZ Wars), and we are not going to field an armoured division with MBTs again, however we will field a combat force with helos, armoured vehicles, artillery, and support vehicles. However having vehicle decks rated for MBTs is a no brainer and it would be part of the interoperability that is required. This would also mean the the flight deck should have a couple of spots rated for Chooks and Ospreys as well.
Me being me, I would also like to see any Kiwi amphibious vessel kitted out with both more, and more overall capable weaponry when compared with how most other amphibious vessels are armed. All-around coverage of the approaches to the vessel by CIWS (preferably something larger and with greater range than the Mk 15 Phalanx 20 mm) as well as an aerial self-defence capability. Given that the RNZN is adopting Sea Ceptor, it would make sense that a 20+ Sea Ceptor VLS be fitted. A naval gun like is fitted aboard Singapore's Endurance-class LPD's might also be a good choice giving both an ASuW and NGS capability and perhaps even more air defence capabilities given some of the specialty munitions now available for 76 mm and 127 mm naval guns.
I am a strong adherent of the distributed lethality strategum and I agree that Canterbury, Aotearoa and any future LHD should have better armament for defence. I would like to see them fitted with at least 2 x 35 mm Millennium guns each and 6 - 9 ExLS VLS cells each which would give a total of 24 - 36 Sea Ceptor missiles. Possibly FFBNW NSM.
Whilst the warfighting side of a defence force is always its first raison d'etre, in NZ's case HADR runs a close second and that is because of our geography, our size and our international obligations. First of all you need to understand the spatial context and where we have to respond each year multiple times for HADR, during cyclone season.
In that map we would respond to HADR within that red polygon, plus areas outside of that if our capabilities are requested. That is a significant part of the Pacific Ocean which has little first world infrastructure, so a LHD with significant air and surface ship to shore capability that is not dependant upon airport and port facilities is a requirement. Such operations do require effective utilisation of all assets and one of the lessons learnt from the Kaikoura quakes was that rotary wing assets are critical and they are not good parked in a hangar or ashore or waiting for flight deck spots to be freed up. That is where the through deck flight deck is better than an aft only flight deck because it allows for greater efficiency in helo flight deck movements.
It is that HADR aspect and now the climate change aspect which will now help NZDF gain support for capabilities like LHDs because they will be seen as necessary assets to help neighbours and friends in the future.
One more point which is important in the NZ context. The Australians looked at LPDs and LHDs and decided upon the LHD. That will be an important consideration in NZG eyes.
Finally analysing this topic in a NZ context purely from within a military amphibious conceptualisation is going to lead to fatal flaws in logic resulting in false conclusions because it ignores the HADR conceptualisation which has similar validity in the NZG priority weightings.
