Your Diablo III regional settings are tied to your residing country in your Battle.net profile, so if your address is in New Zealand, you'll be tied to "The Americas" by default, for both play sessions and the real money auction house:
http://us.battle.net/d3/en/blog/5203654/Introducing_Global_Play_for_Diablo%C2%AE_III-5_1_2012
http://us.battle.net/d3/en/services/auction-house/how-to
Yes, the 7.6 GB retail client is the full retail game, online single player and all. If you were to pre-order the game from Battle.net directly, the download links I posted previously are what Blizzard would offer to you. They are publicly available to download.
Retailers, OffGamers included, probably aren't allowed to sell you the game until 15 May, so it'd be a pre-order at this stage, but regardless you'd have to wait until 15 May @ 7pm NZST for Blizzard to launch the Diablo III servers to the public. You can't even install the game until then, the installer won't let you (the installer's files are encrypted, and will only decrypt the files once the launch date has been reached).
Couple of screenshots for you:
Installer files
Installer window
Trusted
... 
