Best time is June-July if you're a white man who isn't built for the heat (like me).
I echo the previous comments re: a coup in Fiji. A coup in Fiji is safer than a night out in Sydney. Fijians do NOT like to include foreigners in their politics, and consider it very bad form.
Took me three straight nights on the piss with a Fijian Major before he would admit, on Independence day:
"Yes, we are very grateful to the British for our independence. It's just a pity they didn't take their ******* Indians with them when they left".
Lovely people, lovely country - doesn't get any better in the Pacific.
I'd take Fiji over Bali anyday, even with boys in green surrounding Parliament.