yeh im in australia aswell, for my water changes use filtered seawater does the job perfectly
initially however i started off mixing a saltmix bcoz i couldnt afford 600 litres of water straight up to fill my tank
aslong as water isnt collected too close to busy shipping lanes or pollution high waterways it should be sweet.
think mine is sourced by deep ocean trawlers , then sold to lfs filtered and sold for 5 dollars for 25 litres
15 bucks a week but it saves me doing the saltmix , which isnt that hard im just lazy