I'm no expert either, but doubt that wide body jets are the answer for Australia.
If Tullamarine is the only airport it can use in Victoria (okay Avalon too), there are going to be lots of efficiency problems.
As for CL-215's or 415's, we seem to be a bit short of waterways of sufficient size to enable a decent scooping run.
Trackers or Convairs could be an option, but the AT-502 & 802 will always deliver the best bang for the buck in Australia -
- lots of them already here (thus limited additional capital cost)
- plenty of skilled and experienced crews (already spend their days close to terrain)
- can land almost anywhere
- quickest to turn around
It is my sincere hope that we do not ever see another fire season like last summer.