Booked to visit my family in January (smack in the middle of Aussie summer) and was surprised since it costs less per person than my one-way ticket from Aus to London about 2 years ago. Price will probably change over time but definitely the cheapest if you're looking to get out of here over winter.
only downside is the return trip if you don't like planes, airports or travelling in general.
It's Garuda Indonesia Airlines, flying from London to Melbourne. Dates are from 01/01/16 to 13/01/16. One stop on the way there and 2 stops on the way back. Quite cheap for what it is. You'll have to search for the dates above and it should be Heathrow to Melbourne International Airport.