You should include the recipients name (the person you are writing to), their street address or PO Box number, suburb, state or territory and postcode.
The recipients address should be written on the front of the envelope, and should be placed in the centre of the envelope.
And don't forget the postage stamp, which should be placed in the top right hand corner, as set out in the example below.
The return address is important as it means the letter can be sent back to you if it's delivered to the wrong place.
The return address (your address) should be written neatly at the top of the letters seal flap as shown below:
For more letter addressing guidelines including how to include Traditional Place Names, you can check the Australia Post website here.
References
Australia Post. (n.d.). Addressing an Envelope. https://auspost.com.au/content/dam/auspost_corp/media/documents/addressing-an-envelope.pdf
Australia Post. (n.d.). Addressing guidelines. https://auspost.com.au/sending/check-sending-guidelines/addressing-guidelines
Rinck Content Studio. (2020, February 7). Valentine Envelope [Photograph]. Unsplash. https://unsplash.com/photos/rMuBASHufwo