In fact you can do so right up until 16 September. And you can just do it on-line (right now). Takes about 2 mins. Go to the elections website and enter in your details into the enrolement form and you are sorted.
This makes it heaps easier (and quicker) to vote on the day. It also means you get sent info about voting day and who is standing in your electorate, the parties, etc.
Check out details on the elections website overseas page. You can enrol on-line like everybody else, so long as you have been back to NZ in the last three years. You can't vote online though, your vote must be SENT. Voting opens from 31 August for overseas votes and voting papers can also be downloaded. Your vote has to be received (either by mail, fax or by your local consulate) by the 16 September.
Special votes can be cast from 31 August. See the elections how to page.