This developer guide shows how to access Australia Post shipping API. See also the list of required API credentials / keys for Australia Post integration. 

Get Australia Post Developer Credentials

Email Australia Post  customer_connectivity@auspost.com.au and cc support@postmen.com Subject as "Shipping and Tracking API suite", tell them your account number, and contact you to discuss the Shipping and Tracking API suite.
Australia Post will provide Australia Post Shipping API test and production credentials: Password and API Key
If you decide to integrate with Australia Post directly (i.e. not via Postmen), you can read Australia Post API docs online.

Required Australia Post API credentials


Australia Post Developer Resources

Read Australia Post API docs
Supported Australia Post service types
Postmen Australia Post technical support: support@postmen.com