plasterboard on the eaves? sounds odd but maybe they do it different over there.
using a wet area plasterboard on a ceiling sounds odd too.
you sure it's plasterboard in the garage and not hardiflex or villaboard(fibro)
give it the finger test, if you can poke a hole though it it's buggered
in any case it's a piss poor effort on their part to have a swimming pool on top of the ceiling!!!