It seems the official answer is no.

However I use the c&s bags and my own bags as well.

My bags are the same sizer, some (500g) are even slightly smaller... I have not had any dramas with AusPost *touch wood*. Been doing this about 1 - 2 years now.

If going to to use the 3KG bags, make sure to cover up the 3kg writing and ensure your item is packed to around the same size as a 500g satchel.

Having said that.. the c&s 500g and 3kg satchels.. are fairly stretchy as well....

Just ensure your item is within the right weight. If goes over they will send you a bill for the rest....