G'day! ; p
I haven't heard of an issue with sand, wood is the most common trouble item people have. You do need to declare "sand" though, this is what our
customs website says on the issue:
Food, plants, animals and biological goods
Declare all food, plant and animal goods, equipment used with animals, biological materials, soils and sand to Quarantine on arrival. If you don't, you could be given an on-the-spot fine or face prosecution.
Declaring it is usually enough. They might ask you about it, if it seems harmless (ie. beach sand that's been primed, painted and varnished) you'll probably be able to go through fine. It might get stuck in a quarantine for a few months but I think that's highly unlikely:
Declaring goods does not necessarily mean your baggage will be examined.