In the stationery aisle at Woollies, you might be lucky enough to find 52 pieces of Giant washable chalk for $4.99. They are in a green cardboard box about the size of a 6 pack. My sister picked a couple of boxes up from North Lakes, I got one from Morayfield, and just saw a couple of boxes at Taigum. It doesn't say they are on clearance, but 52 pieces of the giant chalk for $4.99 is a bargain. Good brand, and washable, so you can clean up after the kids have played on the roads (not clean up the kids off the road haha)
I'm assuming it will be in every store that has stocks. As a comparison, BIGW have a 12 pack of noname giant chalk for about $7.