One job in Pymble recently had me removing an old air conditioner and patching holes from a disused slow combustion heater.
The flue from the heater went all the way up through the ceiling on the ground floor, the floor on the second level and then the ceiling again to the roof.
After marking and cutting my template for the circles I got to work installing timbers between the joists and rafters so that I would have something to fix my circles to.
I used two different thicknesses of ply to fill the holes, the thicker one for the floor boards and the thinner for the plaster ceilings.
The refrigerated air conditioner was about a foot (300mm) off the floor in the upstairs hallway, probably not the most efficient place for it!
A circle of spare carpet and a bit of painting later and you would never know there were ever any holes.