Mick it sounds like you have a battle ahead of you.
Once it spreads to that degree its going to be hard to get rid off.
I would be hitting it with roundup or maybe ask at your rural supply store or DPI for the best herbicide to use.
I have used roundup to kill different weeds in our dam and know of others who also have ,with no apparent harm to the fish.
I have also heard of a bug that has been used to control it quite effectively,but I think you would have to find some salvinia that contains the bug and introduce that into your own waterway and as the critter breeds it will kill out your salvinia in the least harmful manner.(I'd call DPI and talk to their resident expert).
good luck
Scott