my advice is this
take the opportunity if it is there to visit Fiji. You never know when in your life time you will get the opportunity again.
with your very tight schedule I suggest the following.
forget about doing a trip out to the islands off Nadi because most of the day is taken up with travel to the islands and you just dont have the time.
I suggest the following stay at Danny's Village Homestay and experience the real Fijian Culture
arrive Nadi Airport 2PM
catch a bus from the airport to Danny's this will take about 2 hours so by about 6PM at night on your first day you will be in a real Fijian Village
2nd day experience the Fijian culture and lifestyle in the village with trips to local markets, snorkleing in the bay, fishing, school tours etc
3rd day morning in village and then catch a bus back to the Nadi airport for your 3PM flight back to Australia
see pictures of the village here at this link
http://www.fijihomestays.com/fiji_homestays_photos.htm
lots of photos to show you what you can get up to.
http://www.youtube.com/watch?v=H7Cj7sCshVM
hope this helps