Hi there Tom,
I'm Jon, a Shopify Guru.
Currently, there's no way to have one carrier as the default. That's a great suggestion of a feature which I'll pass along to our developers, but for now, you have to select the carrier from the drop down list anytime you're adding tracking info.
I hope that helps! All the best!
Shopify identifies the carrier by reading the tracking number. In most cases, when you enter the correct tracking number, it should display the right shipping carrier. That's how the algorithm works. But there can be issues since many carriers have similar structures. In such cases Shopify fails to recognize the carrier as Aus Post and displays it as DHL.
As a solution to this problem, you can try the Australia Post Rates & Labels app. The app helps you automate the Australia Post shipping.
With the app, you can print the labels, show live rates at checkout and also handle tracking. When you generate Australia Post label for an order, the tracking number is automatically updated back to Shopify which is then included in the Shopify shipping confirmation email. This reduces the manual effort as well.