Qantas A330 (great for pairs) scheduled Sydney outbound departures at 9:50AM arriving at 4:40PM.
Emirates A380 (larger plane, subjectively quieter) scheduled outbound Sydney departures at 6:30PM arriving at 12:55AM the next day.
Both have overnight inbound flights.
In my opinion, best option would be taking the Qantas to Bangkok, then the Emirates on the return.
Likely cheaper if you book from an online travel agent as mentioned https://www.ozbargain.com.au/node/313340.
OTA's have a mixed bag of opinions, but I have tried https://www.qantas.com/travel/airlines/price-promise/global/... once and it worked out fine for me.
Low season dates:
1 Aug - 8 Sep 2017
23 Oct - 18 Nov 2017
20 Feb - 21 Mar 2018
Other cities, around the same price, connecting in Sydney:
2,600 Qantas Points and 30 Status Credits each way for Sydney departures.
5,200 Qantas Points and 60 Status Credits for the return from Sydney.
+1,600 Qantas Points and +20 Status Credits roundtrip if departing from MEL/BNE/ADL, due to the domestic connection to Sydney.
Calculated under: https://www.qantas.com/fflyer/do/dyns/initialPointsEarned
+1,000 Qantas Points if you take the punt on Qantas' Price Promise.