I did YYZ-MEL earlier this year...
YYZ->LAX (AC)
LAX->AKL (NZ)
AKL->MEL (NZ)
And back...
That said, it was $8500 (and that was a good price too). I am not aware of any possible way of getting to Australia in J for less than $5k. It's reputably bad for that.
You may look at doing it in Latitude or PE, then trying to upgrade out of pocket (eup or some other latitude magic). Do the YYZ->YVR->BNE or YYZ->YVR->SYD trips and do something local to get to melbourne.