I wouldn't. There could be all sorts of pesticides or goodness knows what else in there. I got my play sand from Walmart for $2.56 for a 50lbs bag. I know you guys probably don't have Walmart in Australia, but I bet your local hardware would have it for pretty cheap.
yup, Ruky try Bunnings, just make sure it is pure, untreated sand.
One word of advice, be prepared for lots of cleaning prior to putting it in the tank [as with any gravel/substrate] you want the water to be running clean through the sand before using it. Generally it is much easier to do "handfuls" at a time, than trying to clean a large amount.
Also, just be careful of filter intake tubes, some have been know to suck sand up into them if placed too close to the sand, damaging the impellers, and ultimately the filter.
I highly suggest that the sand you select be new and not from a parks sand box
I also would recommend using Pool Filter Sand instead of play sand. From information I've read, it seems to be easier to clean the pool filter sand in the initial set up. Too, play sand may be a bit sharp for the fish.
You can get playsand from any Better Pets & Gardens. It should cost you under $5 per 20kg bag for the Rich-Gro brand. It is safe for use in aquariums but Pool Filter Sand is better IMO. You should be able to get pool filter sand from Buccaneer Pools & Spas, Aqua Technics & most Clark Rubber stores.
Avoid using sand from public playgrounds. The grass & plants surrounding such playgrounds are often fertilised by local councils & can have all sorts of pollutants in them that may be safe for kids but not necessarily for your fish.