You can find all the rates here
https://www.google.com/voice/rates. The guide says that calls to the US and Canada are free only when calling from the US or Canada. The rate is $0.01 per min from everywhere else. It may depend on where you register from as I was getting free calls from NZ to both Canada and the US but lately calls to Canada have continued to be free but US calls are now $0.01.
The rates for calls to New Zealand are actually pretty good if you don't have a plan on your regular phone.
New Zealand - Landline $0.02
New Zealand - Mobile $0.20
I have found voice quality usually better than Skype. And you can make free videos calls to other users of gmail as you can with Skype.