There are occasionally Jetstar tag flights between SYD and MEL that operate from the international terminal, though see seem to be few and far between.
Otherwise, Air India and United both used to fly between SYD and MEL, but both services have been discontinued and it was not possible to book just the SYD-MEL sector on these flights anyway as neither airline had/has fifth freedom rights.
It would be more like eighth freedom rights as fifth freedom (as defined in the table below) is "the right to fly between two foreign countries on a flight originating or ending in one's own country" an example of which was QF29 flying HKG/LHR which had originated in MEL.
I don't know whether any airliine will permit you to fly domestically with them and have a stopover at the Australian gateway city on the proviso you have an onward flight with them ex Australia eg PR BNE/DRW prior to an onwards flight DRW/MNL or CX BNE/CNS before a CNS/HKG. Also known as own-stopover rights or ICAO's exact definition being "the right to discharge and take on its own traffic that is destined for another city after a stopover."
Another example was LAX/HNL on QF which couldn't be sold in isolation but a passenger could fly on QF LAX/HNL if they were continuing on to Australia HNL/SYD. A current example is flying QF JFK/LAX with a stopover before LAX/SYD on QF.
During the existence of the 'two airline agreement' in Australian prior to its demise in 1989, it was possible to fly on the domestic leg of a QF International flight as long as you had an onwards QF flight on the same ticket eg SYD/PER on QF23 which operated on to HRE if your ticket was SYD QF PER QF SIN QF SYD.
More here:
http://clacsec.lima.icao.int/Reuniones/2007/Seminario-Chile/Presentaciones/PR01.pdf

a diagram of the nine freedoms, with blue circles indicating the operating airline's domestic market and red or yellow circles indicating foreign markets
[TABLE="class: wikitable"]
[TR]
[TH="bgcolor: #F2F2F2, align: center"]Freedom[/TH]
[TH="bgcolor: #F2F2F2, align: center"]Description[/TH]
[TH="bgcolor: #F2F2F2, align: center"]Example[/TH]
[/TR]
[TR]
[TD]1st[/TD]
[TD]the right to fly over a foreign country without landing[SUP]
[4][/SUP][/TD]
[TD]Toronto - Mexico City by a Canadian company, overflying the USA[/TD]
[/TR]
[TR]
[TD]2nd[/TD]
[TD]the right to refuel or carry out maintenance in a foreign country without embarking or disembarking passengers or cargo[SUP]
[4][/SUP][/TD]
[TD]Toronto - Mexico City by a Canadian company, stopping for fuel in the USA[/TD]
[/TR]
[TR]
[TD]3rd[/TD]
[TD]the right to fly from one's own country to another[SUP]
[4][/SUP][/TD]
[TD]Toronto - Chicago by a Canadian company[/TD]
[/TR]
[TR]
[TD]4th[/TD]
[TD]the right to fly from another country to one's own[SUP]
[4][/SUP][/TD]
[TD]Toronto - Chicago by a US company[/TD]
[/TR]
[TR]
[TD]5th[/TD]
[TD]the right to fly between two foreign countries on a flight originating or ending in one's own country[SUP]
[4][/SUP][/TD]
[TD]Doha - Bangkok - Kuala Lumpur by a Qatari company[/TD]
[/TR]
[TR]
[TD]6th[/TD]
[TD]the right to fly from a foreign country to another while stopping in one's own country for non-technical reasons[SUP]
[4][/SUP][/TD]
[TD]Dubai - Cairo - Paris by an Egyptian company[/TD]
[/TR]
[TR]
[TD]7th[/TD]
[TD]the right to fly between two foreign countries while not offering flights to one's own country[SUP]
[4][/SUP][/TD]
[TD]Kuala Lumpur - Jakarta by an Italian company[/TD]
[/TR]
[TR]
[TD]8th[/TD]
[TD]the right to fly inside a foreign country, continuing to one's own country[SUP]
[4][/SUP][/TD]
[TD]Chicago - New York City - Toronto by a Canadian company[/TD]
[/TR]
[TR]
[TD]9th[/TD]
[TD]the right to fly inside a foreign country without continuing to one's own country[SUP]
[4][/SUP][/TD]
[TD]Beijing - Shanghai, by an Italian company[/TD]
[/TR]
[/TABLE]