Flight Companies and Options

My boyfriend and I are looking at travelling for three to four weeks at the end of November/December. We're looking at purchasing our tours from Awesome Adventure for New Zealand (South Island) and Fiji.

I was just wondering if anyone can recommend the best way of getting there and back. We live outside of Sydney. We're looking at flying Sydney to Christchurch, then Christchurch to Nadi, then Nadi to Sydney. OR Sydney to Nadi, then Nadi to Christchurch, then Christchurch to Sydney.

Any airline websites would be very much appreciated! Thanks!