Product:
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.
Pricing:
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:
MEL
https://www.qantas.com/au/en/flight-deals/city-pairs.html/me…
BNE
https://www.qantas.com/au/en/flight-deals/city-pairs.html/me…
ADL
https://www.qantas.com/au/en/flight-deals/city-pairs.html/ad…
Points Analysis:
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.
Just priced up from CBR and it's only $664 return as well which is a bargain.