Not sure whether or not to add this as a comment in support of jpatokals answer or as an answer in its own right, but chose to go with answer.
Adding some personal experience here, as my experience is nearly identical to your intentions:
I arrived in New Zealand on the 29th of June and received the ambiguous "6 month visitor visa" at the border (I use the term ambiguous because it gives you no dates!) - I checked my visa status using the online Visa Verification Service and noted my visa expiration date was the 29th of December.
I left New Zealand on the 13th of August for a weeks visit to Australia, and arrived back into New Zealand on the 19th of August. I was again issued the ambiguous "6 month visitor visa" at the border and let into the country.
I again checked my visa status using the online Visa Verification Service and my new expiration date was the 19th of February.
I did indeed receive a brand new 6 month visitor visa.