From Auspost's website:
Letter size and weight guidelines
We classify a letter as an item that:
In other words, just put a $1.10 stamp on the envelope and stick it in a mailbox, don't give anyone the opportunity to argue.
Incidentally, I'm told that here in the boonies of TAS, you can put your outgoing mail in your OWN letterbox with a red rag tied somehow to the box, and your mail man/lady will take it and send it on its way. I've never done it, but my neighbors have.
				
			Letter size and weight guidelines
We classify a letter as an item that:
- weighs less than 500g
 - is no thicker than 20mm
 - contains flexible items - nothing rigid or inflexible
 - has a rectangular shape
 - is no bigger than a B4 envelope (260mm x 360mm x 20mm)
 
In other words, just put a $1.10 stamp on the envelope and stick it in a mailbox, don't give anyone the opportunity to argue.
Incidentally, I'm told that here in the boonies of TAS, you can put your outgoing mail in your OWN letterbox with a red rag tied somehow to the box, and your mail man/lady will take it and send it on its way. I've never done it, but my neighbors have.