Coinbase is one of the largest exchanges in the world with over 11 million users. One of the keys to their success is simplicity. Once you have set up an account they offer a simple buy and sell functionality at the market rate. More advanced users are encouraged to use GDAX, their full exchange platform.
Coinbase are registered in the United States and hold an active Bitlicense. Unfortunately Coinbase do not offer New Zealand Dollar deposits. Therefore if you wish to trade on this platform you will need an overseas USD or Euro account. This limits the amount of trade that New Zealanders can do on this exchange.