A minority of countries around the world have a voting age younger than 18. Among them, Austria, Brazil, Nicaragua, Cuba and the Channel Islands (Wikipedia reference: http://en.wikipedia.org/wiki/Voting_age).
There is no reason why New Zealand couldn't follow, especially since other rights and obligations of citizens affect people aged 16 and over, such as age of consent, the right to work and the duty to pay tax.
Amendments to the Electoral Act lowering the voting age and age of eligibility are needed.
Sue Bradford had a private member's bill acovering this but has been withdrawn. The bill can be found here.