Okay, I think I got something. Not 100% happy with the price, but I think its the best I can get without mucking around too much. Also cheaper then your London flight
$1262 - $1269 and it puts you in the region you want to go to. You start in Frankfurt and you leave from Amsterdam. Or you can start in Vienna and leave from Amsterdam.
Basically its two transactions. The first is a flight to Bangkok. The second is into Europe and return to Melbourne. There are layovers obviously, but its not really that bad.
1st Transaction:
Jetstar - $308 - 28/7 - MEL -> BKK (jetstar)
Arrive 21:05
2nd Transaction:
Etihad - $961 - 29/7 - BKK -> FRA (kayak)
19/08 - AMS -> MEL
29/07:
Depart 02:50 (no reason to leave the airport)
2.5hr Layover in Abu Dhabi
19/08:
Depart 22:00
~4 hr layover in Abu Dhabi
Arrive: 06:05
--or--
Etihad - $954 - 29/7 - BKK -> VIE (kayak)
19/08 - AMS -> MEL
29/07:
Depart 02:50 (no reason to leave the airport)
2.5hr Layover in Abu Dhabi
3 hr layover in Dusseldorf
19/08:
Depart 22:00
~4 hr layover in Abu Dhabi
Arrive: 06:05
All you would need to do is pay a baggage fee for the jetstar flight to Bangkok and then however you get to AMS in the end.
If you decide to change the fly into city it goes up in price. Frankfurt and Vienna are the best. Flying out city does the same but even more.
Edit: You can go to Jetstar directly to book the first flight. Then just go to kayak.com.au to book the second. Just select multi city and put in the info I said. You can play around with the dates a bit, but not much. I'd also probably book sooner then later if you are serious about it.