Simply put, an amphibious ship is quite different from a humanitarian assistance ship, or a humanitarian assistance-peace support-disaster relief kind of ship. A humanitarian assistance ship, while not quite built to warship standards, is still a complex undertaking, and not merely for the ship. It's a national capability that's delivered and that integrates land, sea, and air capabilities to, in effect, deliver to shore in a foreign country. Each piece of that has its own costs, as does bringing it all together into a package and deploying at an operationally relevant level. At a certain level, it's irrelevant because it's too small. At a level that produces operationally relevant capability, it starts to get rather expensive.
In view of what I've outlined in terms of the shortfall in defence capabilities and defence spending, I think that pushes the discussion of an amphibious capability far off into the future. One needs to have the basic capabilities first before one moves beyond that, despite the fact that Australia has procured two such ships from the Spanish shipbuilders to give them capability in their region.