If you have purchased the digital edition of Diablo III, direct from Blizzard's store, and you then downloaded the install files before May 12th, you may have trouble installing the game. Blizzard note, however, that most Kiwi gamers will have downloaded the US version of the files, which shouldn't have this issue.
That said, when attempting to run the installer you might get an error once you accept the EULA, along the lines of "Unexpected error during installation".
If that's you, there's a simple fix posted on Battle.net:
The folder in question is your main game data folder. If you have the game downloaded to D:GamesDiablo-III-8370-enGB-Installer, please point the download to D:Games. Do not use the automatically generated folder containing the installation files (i.e. do not use the directory D:GamesDiablo-III-8370-enGB-Installer as this is the one created by the downloader).