You should allow anywhere between a couple of days to a week or possibly just a bit more if you live out back of Bourke or something. Even to get a simple birthday card posted to the missus from your place of employment will take two to three days to arrive across a city to your home address and a tad longer out in the country. It should not really, but sadly it does though. I have worked and my brother does work for Australia Post.
Parcels can take longer though as can anything that is larger than a letter in an envelope. Bills always seem to appear almost overnight though.