Hi Sara, i have looked at my special staff industry rates, and i'm afraid the cheapest i can get is via Singapore with BA and QF, and even with the discount it is £942.
Re my above post... China Doll has researched this as she's going at xmas (just in case you missed her spouting off about it!)... she says expect to pay more like £1400 for that time of year...
China Doll is going before Christmas, which is peak peak peak period. They'll have come down a bit by February, but still fairly high as it's summer there. Cheapest routes may be ones that go via a hub somewhere else, like Emirates or JAL or Malaysian. Your sister will need a visa, free online.
Try skyscanner.net. I got my girlfriends sister a flight home from Australia at 12 hours notice for €650 last week, so I'd assume if you were booking well in advance it may be cheaper.