The "Download Now" link directs you to the iTunes App Store, where you must continue the download process. You must have iTunes installed in order to open the link, and you must have an active iTunes account to download the application. This download may not be available in some countries.
For the purchase and display of the Kakadu Park Pass. This allows you to purchase a Kakadu National Parks Pass and have the purchased parks pass to display on the phone for Parks compliance. This is the same as using your web browser to buy Park Passes (tickets). The same levels of security apply. After the Kakadu visit you can delete the Park Pass by removing the app.