To connect Australia Post to Shippo, you need to have a business credit account with them, as well as a parcel contract. Minimum volume requirements apply with a typical requirement of 1,000+ shipments per year and you can apply for a business credit account here: http://auspost.com.au/business-solutions/open-a-business-credit-account.html
Once you have your business credit account, you can find further details on how to apply for a parcel contract (if you don’t already have one) here: http://auspost.com.au/business-solutions/high-volume-parcel-contracts.html
To get access to Australia Post shipping services (via a suite of APIs), you need to register your interest for ‘Shipping and Tracking’ at the Australia Post Developer Centre: https://developers.auspost.com.au/apis and then click on Register your interest today under the "Shipping and Tracking" snippet.
Once you've done this, Australia Post will send an invitation email and instructions on the steps to follow to set up your API key. In Shippo you will need to enter the following:
The Australia Post Account Number, this is the charge account number associated with your parcel contract. It must be entered as a 10 digit number in a format like: 00079xxxxx (10 digits, numbers only).
The Australia Post API Key, which is the corresponding API Key. The API key format for Australia Post is something like: a0fe680d-e298-46c7... (36 characters, numbers and letters, including hyphens).
The Australia Post Password, which is the secret you entered when you created your API key. Your Australia Post password is not the password you use to log into the Australia Post Developer Centre.