So, the most simple option to describe would be to fly:
LHR-SIN-SYD return, BA all the way under the direct flight number. You could use the 241 against the whole thing.

If there is no availability when you want, try the flights as their separate numbers. (On a pure redemption it makes no difference). Again, all BA flights, so 241 against it.

If you can't get straight through availability same day, then try adding a stop over in SIN, to let you pair up availability.
You could stop over in SIN, be flying BA metal all the way, and make use of your 241 on the whole thing.

If you want to go somewhere else for a connection (and stop over) in one of the directions, e.g. DXB/HKG or you want to fly into or out of MEL, then you would need to spend some of your Avios on flights operate by another oneworld airline.

Use of the 241 voucher must originate in your membership country, which I am assuming here is UK?
But you can book an 'open jaw'. So you could, for example, fly BA LHR-HKG outbound, then return to London from (say) Tokyo flying on BA metal NRT-LHR, using the single 241 against the whole thing.
But it needn't be Tokyo (that was a random and probably expensive pluck).

[You could not 'open-jaw' between DXB and HKG because the distance between the open jaw points DXB and HKG is actually greater than the shorter of the two legs.]

If you did something like that, you could then use Avios points for flights with other oneworld airlines, say Cathay Pacific from HKG to MEL (or SYD), or Qantas from SYD to NRT.

That is keeping things simple with a BA open jaw, and two other flights into and out of Australia. Because the 241 would need to be a separate booking, it is good you are having a stop over to protect against flight delays.

But if SIN is one of your stop over options, then you could use the 241 against three of the four legs obligator legs to Australia doing:
BA LHR-SIN(stopover)-SYD
BA ***-LHR (where *** is HKG, or ....).


This is all thinking about things in terms of using the 241.

But if your objective is to minimise cash, then you might also look at options for non-UK departures (where you can't use the 241) to bring taxes down. In that case, loads of non-BA options too.