I just typed out a long reply and its disappeared -
As far as I know, only www.hodgerv.co.nz
make 5th wheels here. One firm will do them as a specialised order and we have several large caravan and motorhome manufacturers.
5th wheels seem to be becoming more popular, possibly as the older generation (me included) decide they want to retire and take to the road in comfort. Until recently I had never heard of them.
Several firms are now importing them from American and Canadian manufacturers. Some have them converted before import, others do the conversion here. I believe the wiring and gas fittings need to be changed and the ones converted by the manufacturer are sometimes reversed so that the door is on the opposite side - to suit the fact that we drive on the left of the road.
Weight and size can be an issue - our campgrounds are often small, our roads can be narrow and very winding. Vehicles which can tow the heavier rv's are very expensive and the supply seems to be limited to the few American utes (pickups?) which are brought in by individuals.
I'm a beginner and this is just what I've worked out so far - it might not all be accurate!