Illegal.

See the rules on the oneworld site -

http://www.oneworld.com/ow/air-trave...world-explorer

Footnote 5 says -

"Within Australia only one flight is permitted between Perth and Brisbane, Cairns, Melbourne or Sydney or between Darwin or Broome and Melbourne or Sydney."

Although the sentence construction makes it a bit hard to understand (is it only one flight out of the first group and one flight out of the second group or only one flight out of only one of the groups?), as you want to do two flights out of the second group (" ... Broome and Melbourne or Sydney), it's not allowed.

The PDF file on the oneworld site describes it as -

Within Australia - only one nonstop/single plane service flight is permitted between the following points:
BNE/CNS/SYD/MEL - PER
MEL/SYD – DRW
MEL/SYD - BME
EXCEPTION:
A. No restriction between SYD/MEL - PER for passengers originating in PER
when in conjunction with travel to/from JNB/BOM/SHA/PEK.
B. No restriction between SYD/MEL – PER for passengers originating in New
Zealand when in conjunction with travel to/from JNB.