The postage cost for parcels with Australia Post varies as per the weight of the parcel.
Every parcel that weighs up to 5 kg and is packed by the sender will be charged as per the flat rate which ranges from $8.95 for a parcel up to 5 Kg to $18.50 for a parcel up to 5 kg. If you are using Australia Post packaging then the postage cost would be calculated based upon the size.
If your parcel weighs more than 5 kg then Australia Post has its own set pricing based upon the size of the parcel. The cost varies as per the weight and dimension of the parcel. They have an online price calculator facility to find an estimated postage cost. If you know the exact dimensions of the parcel then you can use it.