You get what you pay for.
Virgin Australia's business class ranges from $739 to $975 for the 4 hr flight from SYD to NAN.
If you want lounge access then you'll need to fly Qantas who are charging between $974 and $2354. That's an extra ~$230-$1300 just for a lounge at the start of your trip.
Some Fiji airways flights may depart around a similar time to Qantas departures and may get access to the QF lounge but given I've heard of the QF lounge declining QR business class pax, it seems the current arrangement means the QF first lounge retains it's oneworld policy and the exception is just for those on QF operated services.