Just from looking at that I'd guess its way over MPM for YYZ-SIN.
What you could try is something like (or the reverse):
YYZ-CPH/IST-SIN-AKL-SYD and call SYD your point of turnaround, with a stop in SIN.
The return from SYD TPAC is more flexible but still severely restricted. I know the following work 100% without extra connections:
SYD-ICN-ORD-YYZ
SYD-PEK-YYZ
You can't do something like SYD-AKL-NRT-YYZ though. So you have to rule out NZ J long haul.