I'd hope to think the availability of iMac G3's in Melbourne would be otherwise quite good. Even if you can pick up a cheap iMac G3, you could transplant the case plastics and the logic board into it. This is much simpler than removing the CRT and PAV board individually. Unfortunately not in all cases is it a direct swap, so here's the compatible systems.
If your current iMac logic board is a 350MHz or a 400MHz, then you can transplant the board into any one of these (which use the older positions for the chips and heatsinks):
iMac G3 350MHz (with or without Firewire)
iMac G3 400MHz
If your current iMac logic board is 500MHz or faster, then you can transplant the board into any one of these (which use the revised positions for the chips and heatsinks):
iMac G3 500MHz
iMac G3 600MHz
iMac G3 700MHz
However if you find another iMac with a board faster than your current one, you would probably rather keep the newer one and *maybe* transplant your case plastics onto it. Thankfully you don't need to remove the logic board or any electrical components to transplant the case plastics.
If your current iMac logic board is a 350MHz or a 400MHz, then you can transplant the board into any one of these (which use the older positions for the chips and heatsinks):
iMac G3 350MHz (with or without Firewire)
iMac G3 400MHz
If your current iMac logic board is 500MHz or faster, then you can transplant the board into any one of these (which use the revised positions for the chips and heatsinks):
iMac G3 500MHz
iMac G3 600MHz
iMac G3 700MHz
However if you find another iMac with a board faster than your current one, you would probably rather keep the newer one and *maybe* transplant your case plastics onto it. Thankfully you don't need to remove the logic board or any electrical components to transplant the case plastics.