NavList:

Hi Frank,
This is not a detailed CelNav analysis but, still may be what you are looking for. In my early career in the 1960’s and early 1970’s, we had a office manager who was English and had been an administrator in His Majesty’s service in Kenya and other places. He would talk with nostalgia about flying on Pan Am Clippers all day, landing on sheltered water for the night, having the beds folded out and made up, sleeping, and waking in the morning to breakfast and another day of flying.

The near-land routes are described in. https://paleofuture.gizmodo.com/what-international-air-travel-was-like-in-the-1930s-1471258414

Boston to Wellington? Wow! It’s not going to be the shortest route but needing to stick close to land, landing places, and fuel. I suspect Boston, Gander, Greenland, Iceland, Ireland, and then across Europe, the Middle East, Far East, Dutch East Indies, Australia ( or maybe Pacific islands), and then New Zealand. Two weeks or more and, according to the website above, $20,000 to $30,000 each.
Regards, Noell