Unfortunately, I don't believe this is possible with Shopify's built-in shipping methods, since Profiles are designed to be combined. So there is no way for the system to know that customers from Australia should only get a flat price while customers from other countries get a combined price.
If you're open to using a third-party solution, Intuitive Shipping lets you set up individual rules-based shipping methods for any shipping zone, so you could have specific product-based Scenarios that create a combined shipping method for customers in the US, and a 'general' Scenario for all products that only charges the flat rate to customers in Australia.
If you'd like to learn more about how to set this up with Intuitive Shipping, you can book a demo with one of our team members who would be happy to walk you through the process.
Please note: when using a third-party rate calculator, including Intuitive Shipping, Shopify requires you to have their Carrier-calculated Rates feature installed. If you're on either the 'Basic' or 'Shopify' plan, this feature costs USD $20/ month. If you switch to annual billing, Shopify will include the feature at no extra cost and give you 10% off the plan. If you're on the 'Advanced' plan or higher, the feature is already included.