There are 3 different box sizes you can use in conjunction with Click & Send, and they have a 15kg weight limit for each size. The boxes are sold via the Aus Post eBay store, online (at Aus Post), or can be bought at a Post Office (unlike the satchels).

Edit: Here's a link to the eBay store, which has more info about the boxes + pricing etc: http://stores.ebay.com.au/australiapoststore/pages/australia-post-store-boxes