score:3
You are probably best off finding low fares online (google flights, ita matrix, skyscanner, hipmunk, etc.), then confirm the price (for 1 passenger) on the airlines website, and then contact the airline directly to buy in bulk mentioning the specific fares that you have found and want.
I would do this rather sooner than later. December is a busy travel month and it's unlikely to get cheaper over time.
It looks like Cathay Pacific is currently the lowest bidder with reasonable connections and travel time at about AUD$1400 or US$1000 for most of December.