Australia Post has made a string of changes to their parcel postal price structure, with increases and decreases across the board.
The price changes came into effect on September 30, 2019.
For September price changes see here
For Australia Post Charges Guide (MS11 - 1.28mb) see here
As a cheaper alternative you might look at Sendle Courier delivery.
SellBidandBuy has no affiliation or association with Sendle and makes no recommendation as to it's claims, services or performance and takes no responsibilty for same.