Australian running a US business
Do I need a US company to run an online business from Australia where the goods are manufactured, and the customers are predominantly US based?
I your goods are manufactured in Australia and sold in the US you don't necessarily need to form a US company. After all you can ship your products directly from Australia and collect payments using Australian merchant account and/or PayPal.
Alternatively, you could use a fulfillment service such as FBA ("Fulfillment by Amazon"), by sending a container with your products to one of their warehouses, having them itemize and then ship individual orders directly to the clients. In that case you would be required to collect sales tax in several states, which might force you to register a company in the US.
In other words, registering a company in the US would very much depend on a strategy you develop for your business.