Blizzard has confirmed that players in New Zealand will not need to download a new StarCraft II client in order to play on North American servers.
On Battle.net, Blizzard writes, "With the announcement that North American regional access will be provided to those who purchased a Southeast Asia/Australia/New Zealand standard or collector’s edition of StarCraft II, some of you have expressed concerns regarding your ISP download caps. We wanted to let you know that it will not be necessary to download a new StarCraft II client to play in the NA region.
"Instead, we are currently planning for an in-game region selector to be added via a patch at the same time or before NA client access is provided to SEA/ANZ players. Using the in-game region selector would enable you to choose which region you will play in afteropening your existing copy of StarCraft II. If an in-game solution can’t be implemented right away, then alternatives will be provided, such as offering a small file download that would duplicate the intended functionality and allow your existing client to access the NA region.
"Our goal is to ensure that you have the best experience with StarCraft II possible, and for that reason, we encourage players in the SEA/ANZ region to connect to the local Battle.net servers for a lower latency, better gameplay experience. That said, we’re working to provide the additional access described above as soon as possible. If you have further questions about North American regional access, please refer to the FAQ."