Thanks for the replies, no it wasn't a seat eg Corbin. I suspect that in addition to the 'look', it also enabled a cheaper solo vehicle registration in some Australian states. Just wondering if it might have come out of Europe as an after market accessory, but equally may have been home made.
Looked a bit like the one Corbin in the photo but much less bulky. Definitely painted in the body colour, so I suspect it was integrated into the seat base, most likely fibreglass.