I spent two weeks in Fiji (granted, this was back in 2003) and found no good snorkeling off the main island. I wasn't a diver at the time, so I can't comment to that. You will need time to get to another island. I stayed on Waya at the Octopus Resort, which did have good snorkeling (and I'm told good diving), but I don't think homestays are possible and the boat trip did eat up several hours in total.
Given your plan to dive, might it make more sense to stay at a resort that has its own dive outfit, so you can have more bottom time? You might look at doing an internal flight to save yourself time.
Given you are in Australia, why not catch one of those ridiculously cheap Pacific Flier flights to Palau, where you'll have amazing snorkeling and diving and can stay in the capital? You would have to fly out of Brisbane. (Note: if you decide to do this, check about the reliability of the flight leaving on time. Heard some stories from a Palaun about flights being canceled if they didn't sell enough tickets). You can also arrange homestays there.
Edited by: tilos