Toys 'R' Us is the cheapest normal price in Sydney ($32.99 or similar, I think). There are warehouses that sell nappies, etc, in bulk but when we looked into it they were not really any cheaper than waiting and getting them on special at the supermarket, or going to Toys r Us.

Coles and Woolies often have them on special for $32 or less. We tend to stock up them.