If you have multiple domains attached to your Shopify store you might want to detect what one is currently being used. You might need this is if you want to show certain content if the Australian domain is used versus when the American domain is used.
You'll want to look into using the Liquid request.host global property. For this to work you do need to make sure you don't have the option (see pic below) set to automatically redirect traffic to your primary domain. Most stores will want this active, perhaps you don't.
There's a code snippet shown at the link above as well but a slight variation is included below. It's pretty simple but should be more than enough to get you started.
grabbing the Liquid code...