This seems to be the only way to subscribe to the service which otherwise isn't available in NZ yet. The service recently had a 40% price cut and some enticing new games added to its library (GTA5, God of War, Uncharted 4)
You will need a free Playstation network US account to activate it with (to create one, look for a tutorial online). Any games downloaded can be played on other accounts on the same PS4 console.
Purchase the length of time you want. Amazon will give you a code that you can then redeem on your PlayStation US account. You don't need a credit card/PayPal set up to activate!
Streaming PS3 games doesn't seem to work as the latency is too high. Downloading PS2 & PS4 games works fine.