You already know how to get the tickets from SYD to Tokyo. You can buy the ticket from Tokyo (Look for Haneda and Narita) to Milan (also multiple airports) independent (different airlines etc).

There are lots of airlines which provide one stop service between Tokyo and Milan. http://matrix.itasoftware.com is a good starting point to find what is available and how much it costs. If you want to stick with Oneworld (for the miles, even if you don't get much or nothing for cheap tickets) Cathay via Hong Kong, Finnair via Helsinki, British via London, Air Berlin (Ethihad codeshare via AbuDhabi and Dusseldorf). Alitalia flies non stop, should be best with children.