Accepted Solutions
I think the letter issue has been made deliberately cloudy by Aus Post in order for more people (customers and AP staff alike) to assume parcel services are required for some things.
There's actually two types of letters - small and large, though the distinction between a small and large domestic letter is much more clear (i.e. as well as smaller size limits [DL envelope size from memory], a small domestic letter can only be 5mm thick, weigh up to 125g and has to be flexible, whereas a large letter can be up to A4 size, weigh up to 500g and can be up to 2cm thick).
AP make mention of small and large international letters, but previously the sizing guidelines were all the same and the only differences were weight brackets and prices, so I figured the difference probably related more to contents, as they refer to postcards, greeting cards, written correspondence etc as a "small" letter - there's limited info available online to clarify this, but all of the info I've found points to registered international letters only able to contain documents. In saying that, I've never used the service, and haven't ever taken the opportunity to read what's actually printed on the pre-paid envelopes, just what I could find in their guides online which definitely suggest only docs are allowed, particularly as they state no customs dec is required for the pre-paid envelopes (which is only applicable to docs).
As many people also know, on the 18th, AP tried to restrict all international letters to written correspondence only, while everything else, no matter what it was, how big / small or light, had to go as a parcel - they rescinded that on the 19th, but knowing AP the memo probably got lost on the way to some places. 
