https://community.ebay.com.au/t5/Selling/Using-old-unused-Prepaid-Auspost-satchel/m-p/2265647#M190302Thank you for your wonderful contribution to this seven year old topic.
Just for the sake of it however, yes, you can still use the old 500g/1kg/3kg/5kg satchels, and these days you can actually shove up to 5kg in any of them ...