Just thought I'd let you know that you are free to come and go out of NZ as much as you please on a WHV. However, if you do visit Oz be sure to take your email confirmation of your WHV with your conditions on it as Oz checkin staff are very strick in letting you leave without an onward ticket out of NZ we found.
Even although we had our WHV stamp and letter of acceptance for PR it took ages of checking with supervisors to see if we could fly out of Oz into NZ. OZ and NZ immi are tightly linked and will stop you from entering NZ unless they are fully satisfied.

We got told at checkin we wouldnt be allowed to leave Oz on the WHV stamp in our passport unless they could see the conditions of our visa! We got there in the end but it's not what you need.

It was all a bit strange as 4 months earlier we visited the UK and left there with just our passports and no documentation of our WHV conditions but there you go. So if you want an easy travel be sure to take EVERYTHING with you.