not sure about Australia, but in NZ we use sea water if its closer, and a high risk fire. In Nelson this year a fire was lit among some houses, and we had 5 helicopters running circuits out of the local harbour using sea water. It was really close to houses, in fact at one stage we were watering the deck trying to put the fire out underneath it, but ended up saving the houses.
I haven't been back to see what the seawater has done to the ground, so cannot report the results, but not too many people were complaining that day.
One consideration could be what the salt water does to the tanks that they use. Replacing that because you used sea water would eat into your profit for the year.