The rules vary between airlines and also according to the type of ticket you purchase.
For example, Qantas have 13 different fare types between the UK and Australia. Even within the 'economy' group, there are 7 different types. So, for example if you purchased 'fully flexible economy' tickets, you can get a full refund, if you purchased 'dreamtime economy' tickets you'll lose �300 per ticket or if you purchased 'sale economy' tickets, you won't get a penny back:
http://www.qantas.com.au/regions/amadeus/minir ules/UKAUFULFLX_SUMMARY/index.html
You'll need to check the relevant airline's website or (simpler) phone them up.
Chris