'this distance is measured from the point where aircraft first reaches a height of 50' above the landing surface'
As stated, this is how it is measured. Now reread this bit:
an aeroplane must not land unless the landing distance available is equal to or greater than the distance required to
bring the aeroplane to a complete stop
What's the definition of "landing distance available"?
"... the distance specified by CASA as being the effective operational length available for use by aircraft for landing at Government or licensed aerodromes or the distance available for landing on an authorised landing area .." So, for a licensed airfield you must get that distance from the good book and the distance required from the POH gotta be less than that. Some room for debate for an ALA but not a lot - if an area is not available for landing then it is not included in the distance available for landing.
As some-one previously stated, we're discussing the rules for less experienced pilots. I know an experienced pilot who operates 3 aircraft from his 450 m one-way strip quite happily.
A student asked to go in there with me on a nav exercise. After checking the POH for the PA-28 he still wanted to go. I declined and took him to a nearby unlicensed strip which was much longer. Even that caused him some grey hairs.
We have lots of places for long runways in Australia, so personally I don't see a need to go anywhere with marginal distance. If I was flying a Husky I'd be happy to land almost anywhere though.