FarmerPete,
I used to live around the Grampians where there were quite a number of suitable lakes. Of the numerous large lakes in the area that would be suitable Bellfield & Rocklands in particular have large areas of deep water even during drought periods. The closest airport with a chance of taking a DC10 or 747 to these lakes would be at Avalon - over 100nm away.
I also used to live in far East Gippsland where there were quite a number of protected Estuaries that would have been suitable for water scooping - including both the Mallacoota Lakes and the Gippsland Lakes. Again, the closest airport with a chance of taking a DC10 or 747 would be Canberra - again over 100nm away.
There are many other waterways in Victoria that I can think of that would be suitable for scooping.
A large capacity tanker will take a significant amount of time to refill on the ground - scooping only takes seconds.
as our lakes are man-made and contain much flooded timber, safety considerations would require the aircraft to only scoop from carefully surveyed and marked runs
I'd be more worried about damn power lines than underwater timber. Having done some flying in an impoundment with both I know which is easier to spot on an aerial survey. Man made marks are usually only needed in very confined areas with hazards that are completely submerged as there are usually plenty of natural features to use as marks. In Australia we are fortunate that we don't have anywhere near the amount of mobile timber that they have in many overseas waterways - seeing large trees floating just under the surface sends shivers up my spine.