This can be achieved in two ways:
- By creating a multiregion website
- By creating a multilingual website
Option #1 will need to detect the visitor/user's country based on his IP and then the content will be served. However, as you have mentioned that you do not want to do that, the option for you is #2.
You can create multilingual WordPress site by using a plugin like WPML. It allows you to create separate content for different languages. Currently you are working for Australian market and expanding to US market. Though the language in both countries is English, in future you may expand to some other countries which have different languages where WPML would play a crucial role. WPML will allow you to create subdirectory or subdomain option for different language sites.
In case of subdirectory your URLs would be:
In case of subdomain your URLs would be:
So in my opinion that would be the right direction.