I don`t think it makes a lot of difference these days when you fly long haul. I would have thought the beginning of their winter would be a bit cheaper (our spring) and if you want to fly over Christmas/New Year the flights are quiet so you would probably get a deal
I would have expected the Christmas/New Year period to have been when Brits would be flying out to Oz for a sunshine Christmas with their relatives, so I can't see that it would be particularly cheap to fly then.
As 237SJ suggests, there's no particular time of the year for the cheapest flights. However Air Asia are currently offering good fares for travel in February and March of this year, from Stansted via Kuala Lumpur:
http://www.airasia.com
Chris, it depends how close to christmas you want to go. Most people go out a couple of weeks before but if you leave it until christmas eve or just after christmas the aircraft will be empty.
Thank for the answers. It would be to visit boyfriends family who he misses terribly. Probably couldn't afford to go as soon as March but perhaps it would be worth looking into Xmas eve flights for this year.