FF, if you are serious, he best place to clarify those issues is to contact the NZ Consulate nearest you. Can't help you re where that would be, but there are some in the US. Of course a lot of that sort of info in general terms is available on their web-sites via good old Gargle.
The really important thing is whether you would be able to exist financially as probably the NZ Pension or National Super, as they used to call it and maybe still do, would probably not be available to you - heck, I'm not sure if even I would qualify now I have been here in Oz for 25 yrs, but I was 42 when I left there, so probably I would, at least in part. The same would apply to the Aussie pension.
The other main issue, assuming there is no other bar to you moving there, (or here), is health cover, and how that would play out, and that is where direct contact might help clarify. You would probably have to privately insure, but you are used to that anyway I guess. We have a Medicare, but if you are eligible for that over there, I am not sure if there is reciprocity with the US version here like there is for the UK, and a few other countries. Even NZers don't get immediate access to Oz Medicare now if new arrivals. A lot has changed just in the time I've been here.
Then there is the question of whether it is ever wise to uproot yourself at this time of life and leave everything familiar, and everyone you know, family as well, (including grandkids), and retire to a totally new place, where you know no-one, even if it is paradise in some eyes.
If however, in spite of all obstacles, you wanted to go ahead, then the choice would come down to what you hoped to gain. What your really keen interests are, and your main love-to-do's in life. Both countries have lovely cruising grounds, but Oz is warmer, but experiences more extremes of weather, NZ cooler, but not as cold as a lot of the US, and has amazing lakes. Aussie has the Whitsundays, Sydney Harbour, Pittwater, Myall and Gippsland Lakes, and NZ the Bay of Islands, Hauraki Gulf, Lake Taupo and a host of beautiful South Island lakes. However, if I could wave a magic wand, the place I would like to be transported to, with my boat would be either the Marlborough Sounds near Picton, or the Patersons Inlet on Stewart Island. That place is a boating paradise, and even though at the foot of the South Island, it is warmed by currents, so that it is more temperate than the southern part of the South Island. It was featurd in a PMM issue of a year or so ago by the family traipsing the world in their Nordhavn - can't remember the name of boat or family sorry, but it is a beautiful and largely unspoiled place.
What has got you even considering this? I'm intrigued.
However, for your information....
MARINE LIFE
https://www.google.com.au/search?q=...kDKXeigeypYCoAQ&ved=0CGwQsAQ&biw=1280&bih=607
https://www.google.com.au/search?q=...fLanniAf8tYHYAw&ved=0CHAQsAQ&biw=1280&bih=607
Bay of Islands Maps - Maps of Bay of Islands New Zealand
Things to see and do in Picton, New Zealand
Picton Marlborough Sounds New Zealand - Information, Guides, Accomodation
Marlborough Sauvignon Blanc Wine New Zealand | Marlborough, New Zealand
Kia Ora, welcome to | Stewart Island - Island of tranquility
Stewart Island New Zealand Travel ~ Stewart Island NZ Information
https://www.realjourneys.co.nz/en/destinations/stewart-island/
But seriously, the boring stuff at the top is still the most important to consider...changing countries does cost, I can vouch for that....