Cathay Pacific, has a 6-day seat sale to Sydney and Adelaide, with return flights on offer from an unbelievable £529.
Flights depart from London Heathrow between 20 April and 11 June, 2009, and again from 1 and 30 November, 2009. Note that November seems to be around the £545 mark though... can't find any for £529 in November, but lots from April to June.
The fares even include one free stopover in Hong Kong (must be booked by phone on 020 8834 8888).
Seats are limited and you need to book by 25 February, 2009.
You can also fly to Auckland, New Zealand, from as little as £534.