So does every one else including the USN & USMC. It's horses for courses, so I strongly suggest have a good look at what each of the two platforms are specifically designed to do before gobbing off.
Gobbing off fair dinkum, I suggest you look at the current requirements for the ADF are they testing an Amphibious Assault Vehicle for land 400?
So far in Australian armoured troop carry vehicles have had the capacity for an amphibious vehicle is to swim not surf, M113 had the capability to which they lost in the upgrade, its not a priority at the moment. Land 400 has prioritised capability of protection, lethality and mobility over Amphibiosity . The Australian Army is no longer built a light infantry force nor is it heavy force either. Terrex 3 was submitted for evaluation for phase 2 which lost out to boxer Terrex 2 was not put up for Land 400 which did have ability to swim in the surf, but wasn't submitted because Army had prioritised protection, lethality and mobility.
We have all acknowledged that the ADF will most likely not land on a contested beach, but we need the capability to move over a beach if port infrastructure is not available. Currently if all three vessels are used the ADF will have the capacity to move and land 18 land 400 vehicles on the LLC in low sea states (higher if replaced by MSVL) plus another 140 troops on LCVP, and if we are pushing all 3 ships into a landing the shit has really hit the fan.
Yet I stand by the earlier claim, that in an Australian context that landing craft LLC(MSLV)+LCVP will get troops and equipment to the beach faster than an AAV, but the less protected AAV will have more simultaneously. Every one likes to compare to the USMC, but we are not the USMC. Until the ADF increases our Amphibiosity into something like a brigade level permanent capability I suggest an AAV is of low priority, just like the F35B on the LHD.
Australia may support a coalition beach landing where mixed land fleet may comprise hosting the ability but we wont be going along to the beach in Australian AAV's anytime soon.