I found a local landscaping supply company sold sacks of washed and graded silica sand, it was like 20Kg for $20 or so...this is in Cairns FNQ. There's a few silica sand mines, we get ours from Sunstate at Bundaberg, washed and graded and optionally kiln roasted if being used with resin. http://www.sunstatesands.com.au/
Our local source of silica sand at Cape Flattery is no longer available, it was super pure glass making grade so the landscaping supplier was sourcing it in 20Kg sacks from down south somewhere. Fraser Island is no longer mining it since the 1970's, they would have been close to Caloundra.
I’m just surprised that Sibelco Incast 50N is so hard to find here, I was thinking it was Qland based but no one seems to have it.