The Defence Capability Plan 2019 calls for a second enhanced sealift ship, it states "An example of the class of vessel that could be considered under this project is a Landing Platform Dock." the important words here are "example", "could" and "considered". This means that they won't be just looking at LPD's they will also be looking at small LHDs or LHA and the Endurance 170 meets all the requirements of what the DCP - 2019 set out and adds more and while we don't know the price of this vessel, the initial indicative capital cost of "More than $1b." is most likely in its price range. Being built in and by the Singaporian shipyard ST Marine, this vessel would most likely not be that much. "More than $1b", is a heck of a lot money for an LPD, for that much you might as well get the LHD/LHA and have the added extra enhancement of the flight deck for various operations.

We also know that around the same time The Republic of Singapore Navy (RSN) is also known to have a future requirement for a large amphibious vessel to supplement it's existing, ST Marine-built Endurance-class of landing platform docks (LPD). The RSN operates four LPDs of this class displacing 8,500 tons. (Endurance 140) However ST Marine is likely to face fierce competition from international shipyards. If New Zealand plays it's cards correct we could tack in on this build maybe getting the second and fourth vessel (The fourth vessel to replace Canterbury). Lowering the cost of these vessels even more. 

So the DCP 2019 says;

• Recognising the high value of sealift to humanitarian and disaster relief, and the sustainment of deployed forces, in the mid-2020s an additional sealift vessel will be acquired. Operating alongside HMNZS Canterbury, this acquisition will provide two sealift vessels, and will greatly improve the effectiveness of the Defence Force, and the resilience of the nation, and the region.
• The enhanced sealift vessel will have greater lift capacity than HMNZS Canterbury. The capability will provide a highly flexible military asset, including hospital facilities, planning spaces, and self-defence capabilities. It will also provide support for the deployment of a range of capabilities, including Special Forces, Unmanned Aerial Vehicles and NH90 helicopters. The enhanced sealift capability will also improve the New Zealand Defence Force’s amphibious operations. Through the provision of a well-dock, it will be able to conduct operations in a wider range of sea conditions and will have the size and capacity to carry large equipment, and sufficient aviation capacity to allow extended long-duration operations. Its size will also provide for the transport of a larger number of personnel, allowing for the value of the increased size of the New Zealand Army to be realised.
• Collectively, these enhancements will significantly increase our ability to respond to humanitarian and security events in the Pacific region. While a future project will determine detailed requirements for this capability, a Landing Platform Dock is an example of the type of vessel that will be considered.
• Following mid 2030's, HMNZS Canterbury will be withdrawn from service. At this time an investment will be made to further improve the Defence Force’s sealift capability with an additional vessel. Options will be explored against the composition of the fleet, the wider Defence Force and the prevailing strategic environment.

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This is borrowed from a post on DefenceTalk (slightly edited)
A Singaporean built mini-LHA would give good value for money. When compared to the Endurance Class 140, the "Joint Multi Mission Ship" built for NZDF will:

• Be even more versatile when configured with standardized mission modules to take on a wide spectrum of operations, including additional modules to support search-and-rescue operations or be deployed with unmanned systems for surveillance or mine countermeasure operations;
• Improve efficiency in logistics and engineering support. In "designing the support", the JMMS' operational readiness will be enhanced as less time will be required for maintenance of the ships;
• Re-use and improve the sense-making and decision support systems used in other classes of RSN ships, like the LMV or MCRV, and also complemented by the already high levels of automation in Singapore ships. This will enhance situational awareness and accelerate decision making, and
• Be equipped with advanced integrated communication and network system that includes tracking of the ship's equipment and logistics status as well as crew movement.​

Why do we need a second Sealift vessel?
The demand placed on the multirole vessel HMNZS Canterbury has highlighted the criticality of effective sealift, while experience accrued over the last decade has highlighted the operational and environmental limits of HMNZS Canterbury, as well as the risks inherent in maintaining a sole vessel of this type. Having a single high demand vessel leaves very little room for maintenance, training, crew leave meaning crew turn over is higher which in turn you need more training for the new crew.

Having 2 vessels of similar capabilities means we (NZ) can respond to multiple emergencies at the same time or still respond if one vessel is in a maintenance period. This has been the issue of only having 2 frigates, and we need to get back to a four frigate navy, as the current ANZAC's have been over worked. They have ben upgraded but they are still worn out. The same thing is happening to HMNZS Canterbury with being the only ship in her class in the RNZN fleet not enough time for maintenance. Having a second will alleviate this issue.

Having the two vessels means the NZDF can move more equipment at once in the time of need whether war, peace or HADR etc. and be more effective at doing the the job they have been given, as well has the troops on the ground have the support they need from the vessels at sea.