I'm heading over to help, my dream job is as a party planner LOL

Could you have it at one of DS's grandparents house instead perhaps? Any family members with a decent sized back yard?
As for an external venue - a playcafe - but a toddler friendly one?
Not sure where you live in Vic but last time we were there we went to the Diamond Valley Railway and it was awesome - I'm pretty sure you can have parties there too (a BBQ or Picnic - an easy picnic is cold chicken, salads and sandwiches as you can do it all pre-party rather than worrying about cooking a BBQ, much easier and more relaxing as hosts
Botanic Gardens? Aquarium?
Find a great playground with shelter/toilets/fenced in playground - I'm sure the Melb locals could fill a thread with playground recommendations
We have a plaster fun house here - there would no doubt be an equivilent (eg an art type of place)
Or a kids circus - sadly a place called Triskills just closed here, but DSs childcare used to take excursions there all the time, lots of trampolines, foam pits, balance beams, swings etc - is there a similar place?