I had a look. And you know I normally agree with you. But you're wrong here.

It's not a normal disabled toilet that just someone in a wheelchair or something needs, with just maybe 2x the space and a rail like you see everywhere

This thing is for your quadapligic and cerabal palsy type cases needing full care kind of patient, who needs a hoist. Who needs someone hoisting them up, and washing them. (which, after a flight...). These take more space.

If there's the space, and if you get a number of people at that level of disability (and then needing to fly elsewhere in aus, as melbourne is somewhat of a flight hub to get to other cities) then it absolutely makes sense. For your comfort, as well as theirs.

The 'all genders inclusive' toilet down the hall is a travesty. But it's not all that odd for australia's major travel hub (well, melbourne and singapore) to have a bathroom and toilet that can cater to people who need a bit more care than just a bit more space and a rail. If anywhere should have this, it's melbourne airport.

E: I checked. Singapore has the same. https://www.changiairport.com/content/dam/cag/airport-guide/special-assistance/assistance-for-people-with-restricted-mobility/Adult-Changing-Room.jpg (though theirs looks a little nicer).

Let's read what melbourne has (and I found photos on blogs):

https://www.melbourneairport.com.au/getmedia/5ff38519-f0cc-40b2-a1cb-1a2699c20e38/Disability-Access-Facilitations-Plan_Dec2020.pdf.aspx?ext=.pdf

Adult change room An adult change room facility is located in T4 behind McDonald’s on level 1. Our adult change room has an adult-sized changing bench, a tracking hoist system, adequate space in the changing area for a traveller with a disability and up to two carers. It also includes a centrally placed toilet with room either side for the carers, a large waste bin for disposable pads and a non-slip floor.

It's a 3 person room. The kind of place for a person who needs up to two carers with em.