You could, subject to availability, fly one way with miles from one ffp and the return with miles from another ffp.

For intra Australia flights you would want to carefully check the cash fares vs award with Au taxes. Some local flights are not that expensive. Other like SYD/MEL/BNE to PER can be *** expensive.

Airlines that fly in Australia domestically
- Virgin Australia (Delta partner)
- Jetstar (a low cost low quality carrier with low-medium-high priced fares owned by QF)
- Qantas [OneWorld Alliance](AS partner)
- Tiger Airways (a low cost low quality carrier with low fares part owned by Virgin Australia)
- Regional Express Airways (REX) Regional airline
- Airnorth Regional airline in northern Australia