Ironically the Tarawa class LHA design was offered upas a possible Melbourne replacement in the 1970s, now a pair of them to replace Melbourne and Sydney would have been something, especially if the Skyhawks were replaced with Harriers.
Imagine the Fiji coups if the RAN had these, the intervention in the Solomons, Australian participation in the 91 Gulf War and the War on terror. Not to mention the many occasions where they would have been a game changer in a humanitarian response.
We have always needed that capability, but Tarawa class is still big and man power intensive. The RAN would have struggled to acquire and operate 1. Time frame of the builds I would imagine would fit, but with an Invincible on offer at discount rates why not go with her (until the Uk wanted her back).
With our two LHD's, we should be able to surge a capability not far short of one american Wasp class LHD. But its that type of capability we have really missed.
Any operation in our region is done with crumbling infrastructure and insecurity. If we had to go into East timor (or Samoa or PNG etc) today with 1000-2000 troops, there would be problems with sanitation, clean water, fuel etc. Local hospitals struggle to cope with existing peace conditions. Security even in peacetime can be problematic. So even a peacekeeping or humanitarian mission is effectively impossible with what we have with a dicey logistics train that would mean we may have to pull out if a cyclone/storm approaches that may damage port facilities.
Throw in any sort of opposition or conflict and there are very limited range of options open to Australia.
We have had long running security and humanitarian issues in East Timor, PNG, Fiji and Samoa. You can pretty much book a LHD at each of these countries elections, in the case of Fiji, elections will only occur once the LHD is operational. I think GF even joked that DFAT could end up paying for the LHD as they will be very interested in their use. Having two 30,000 ton floating islands of Australia that can move around the region is going to be tremendously powerful. In the countries I've listed, its most likely enough to keep things in check just by being present so citizens don't have to worry what the Police or the local military are backing (and if they are backing opposing sides).
By being active in the region they can start building real goodwill towards Australia as we will make a significant presence. Aid/Disaster relief will move into a new level, I would imagine some of the cash aid will decrease in favor of more specific aid administered. But its the soft stuff that going to have influence. International training exercises will also move to a much bigger spectrum, with many more involved in greater depth.
And that's just in the local region. Internationally puts us in a position to make meaningful contributions to US and UK lead forces. To participate globally. When deployed with a UK carrier, Australia might supply the entire amphibious capability for a mission for example (training). So they scale up to major conflict
Spain originally called it a Strategic Projection Ship (BPE). That is a much more accurate explanation of what they are. They aren't just sea control, they aren't just an amphib or a landing helicopter docks. They are big enough to influence regions and small nations.