If you prefer to stick to QF metal/numbers as much of the way as possible (which is obviously preferable if earning SCs is the main objective), the obvious option is to fly to LHR with QF and connect with BA to LIS. I see that BA only fly to OPO from LGW... You could always fly QF to DXB and EK (with QF number) to LGW, connecting to that BA flight.
Flying via MEL (connecting to QF9) will get you a few extra SCs (40 each way in business) without adding much, if anything to the cost. Splitting up the journey in DXB may also help with SCs as you'll earn separately for the SYD-DXB and DXB-Europe sectors.
You may also want to consider flying to MAD via DXB. QF codeshare with Emirates on flights to Madrid. Then, jump on Iberia (earning more SCs) for the ~1 hour flight to Porto.
Another option could be to fly on QF to DXB then onto FRA with EK (QF number). From Frankfurt, fly to Madrid on LAN in Business class. (This yields 60 status credits each way, from memory, and is usually very reasonably priced - though this flight is only once daily and in the evening.) Then, just jump on one last flight to get to Porto. This is a bit of a longer routing, but will get you some extra status credits for not too much cost.
If you want to save money, consider buying separate tickets between Sydney and Bangkok, Bangkok and Madrid, and finally Madrid and Porto. SYD-BKK can be done on QF (or Emirates with QF number). Then, you could fly onwards to Madrid via Doha with Qatar, Amman with Royal Jordanian or Helsinki with Finnair for a very, very good price - while still earning SCs as it's with oneworld. Once again, you'll then need to use Iberia for the final sector to Porto.
The reason I keep suggesting flying via Madrid is because its the only destination (other than London-Gatwick) that is served by a oneworld airline out of Porto. You could also consider flying to Lisbon, or flying via another European port and connecting on Ryanair or easyJet. Ryanair fly twice daily between Barcelona and Porto, for example, and charge very low prices.
