iOS: Apple has its own Cards app for sending actual postcards using photos from your iPhone, but $4.99 is expensive to send a postcard, especially within Australia. Australia Post's rival Postcards app performs a similar function and charges a lower $1.99 for sending to Australian addresses, or $2.99 for international dispatch.
The one advantage that Apple's app has is the ability to charge the postcard to your iTunes account; for the Australia Post option, you'll need to use a credit card or PayPal. But at less than half price, it seems a good choice if you want to impress a luddite friend with a physical postcard.
Australia Post Postcards [iTunes App Store]