In Send SMS (https://dev.telstra.com/content/messaging-api#operation/sendSms) the notifyURL description states: "Please refer to the Delivery Status section for more information."
In Get SMS Status (https://dev.telstra.com/content/messaging-api#operation/retrieveInboundSms) it has the following
"SMS Status with Notification URL
When a message has reached its final state, the API will send a POST to the URL that has been previously specified. Upon receiving this call it is expected that your servers will give a 204 (No Content) response."
However, none of the example responses or documentation mention the messageId, so I had to run some tests to determine what the JSON format of the above mentioned POST to the notifyURL for message status
Hope this helps others, and hope the TelstraDev team update their doucmentation to include it.