If you're prepared to endure Jetstar in economy for eight hours each way (or if you're lucky Thai), TK still has very good value mixed class fares out of MEL. They run to $4.7k-$5k, and are a lot cheaper than starting in SIN. Not as cheap as EY's fares in this thread and you'll be connecting onto TK's dated A330 or (for now at least) 777 business class, but it does offer a protected connection through SIN or BKK, and you can use SQ's excellent lounge in SIN.
Having booked this before, and assuming you're stopping over on the way over (reverse the gap in flights in IST where stopping over afterwards), you need to:
- Check you're flying into an eligible country here: https://www.turkishairlines.com/en-int/flights/stopover/
- Book three flights using the multi-city booking tool: MEL-IST, IST-MAD (assuming you're flying into Madrid) and MAD-MEL. You need to leave at least two nights between when you arrive in IST and when you fly to MAD, so check the timing of the flights carefully. (EDIT - the 'stopover in Istanbul' tool automates this process by inserting the necessary time in IST - just select whether you want the stopover on the way over or way back and the number of days.)
- Send an email to [email protected] with your name, PNR, ticket number, desired accommodation dates, preferred room type, telephone number and email address.
Note you'll need an eVisa to enter Turkiye as an Australian citizen. Good luck!