Adioso has detected a bunch of sales from Jetstar japan. These can be used as cheap return legs for flights from australia. This has not been announced by Jetstar Australia, as far as I know, but the flights are available and bookable.
Details as follows:
Narita to Sydney $193
Osaka to Sydney $203
Narita to Melbourne $204
Osaka to Melbourne $214
You can see a summary of the return legs here:
http://adioso.com/search/japan-to-australia.html
NOTE: Although these are sales in the japan->australia direction, they can still be booked as part of a normal return ticket from Australia to Japan.
If you look at:
http://adioso.com/search/australia-to-japan.html
You will be able to see the normal outbound prices - the return leg is the bit on sale. ie:
Sydney to Osaka ($428) + Osaka to Sydney ($203) = Return for: $631 (pretty good!).
You don't need to do anything special on the jetstar site - just book as per normal - you'll just see that some of the RETURN flights only are about half the price of usual.
Also, if you like, you can watch these fare changes too as they happen by going to: http://adioso.com/feed/au (choose airport if you like).
Happy hunting!
Fenn.
Update #2: More edits to the description of the sale for better clarity. Sorry for the confusion all.
You realise that these are FROM (and return to) Japan and not TO Japan, right?