Rhyb I think flats will be smart, because they will dry quickest - I know in Cairns drying's not the biggest of issues, but I think it'll still prove to be a bonus.
As for the OSFM's, what do you like already? I think if it was me I'd be getting whatever decent thing is cheapest...I'm actually thinking they'll be in flats/really small nappies for awhile, since as triplets they're likely to be smaller than average at birth. So you may have quite a bit of time to decide what you want.

Helen I don't dry inbetween for prewashes.