Mobile phone numbers begin with 04, followed by eight digits. e.g. 04yy yxx xxx. The y-digit codes are allocated per network, although with the introduction of number portability, there is no longer a fixed relationship between the mobile phone number and the network it uses. New numbers are still allocated to phone companies in blocks, so a new number will generally still be on its "home" network.
Within Australia, mobile numbers must always be dialed with all 10 digits, no matter where they are being called from. 04 is a prefix, not an "area code", as such.