Here's a quick tutorial for those wanting a US PS+ membership :)
If you want a US PS+ membership and want to play online using your existing AUS account you will need to setup a secondary US account using the US PlayStation site:
https://www.playstation.com/en-us/home/
There you will need to enter a US address and use one containing a US state that does not have sales tax such as one of the Portland Universities:
PO Box 751
Portland, Oregon
97207-0751
United States
Then you need to purchase a US PS+ membership. I can vouch for this seller, bought 2 years worth for $84 ($42 each) when they had them on special:
http://www.ebay.com.au/itm/121345728193
The seller will send the code to your eBay messages.
Login to your new US account on your PS4 and set it as the Primary account in the settings.
Enter the US PS+ code purchased from eBay by using 'Redeem Codes' down the bottom in the US PlayStation Store.
Now if you want to purchase anything from the US PlayStation Store, you will need to add funds to your PSN wallet.
Go to Amazon.com and make sure you use the same US address as stated above to purchase these:
http://www.amazon.com/10-PlayStation-Store-Gift-Card/dp/B00G…
Redeem them the same way you did the PS+ membership, logged in to your US account on your PS4, and input within the US PlayStation Store.
Now you can switch back and forth between your new US account and your current AUS account and enjoy the cheaper benefits of the US PlayStation Store whilst still being able to play online using your current AUS account! :)
This should also work for PS3 and PSvita.
Thanks