Each purchase order has a delivery address where the purchase order is sent.
There are a couple of shortcuts available when editing addresses.
By clicking on the Lookup button to the right of the Building field, a pop-up window will list any Delivery addresses for the account. Click on an address and it will be loaded.
If you want to lookup a post code, click on the Lookup button next to the post code. If you've already entered a suburb and state, the post codes for that suburb will be shown in a pop-up window. You can also search for a post code in that window. Click on the relevant suburb and post code listed to load that into the address.