"You have to keep remind yourself just how big a shift this is actually," he said.
"We used to spend a lot of time as a country sitting around thinking of way to get their attention to get invited to things.
"Every time we got noticed somewhere, someone would dash out and put out a press release to make more of it than anyone should, none of which was very good for us.
"The whole thing has matured completely in the past eight years."
The warmth of the relationship is such that it is easy to forget how rare visits by Secretaries of State actually are.
The most recent visits were by Hillary Clinton 2010, Condoleezza Rice in 2008, and Madeline Albright in 1998.
The one before Albright was by George Schultz before the suspension from Anzus (the 1951 Australia, New Zealand, United States Security Treaty) and formal reprisals were adopted in 1986.
Kerry has a personal history with New Zealand, having spent time here in 1968 and 1969 as a naval officer on an R-and-R break from Vietnam War duties.
He became an anti-war activist and testified against the conflict before the Senate Foreign Relations Committee, a body he later chaired.
He took over as President Barack Obama's foreign minister from Hillary Clinton, allowing her to concentrate on her presidential bid. He continued Obama's mission to engage more with other countries, to build coalitions, to be less the global policeman, and to put greater emphasis on multilateral institutions such as the United Nations.
Kerry's affection for New Zealand goes well beyond the good times in Wellington in the 1960s.
He greeted McCully today with a hug; all the more pointed because they had seen each other only three days earlier.
At his press conference, Kerry expressed genuine pleasure at the level engagement he encounters with New Zealand across a range of areas, and not least over Security Council business in the past two years.
He and McCully are discussing what the Security Council could do to preserve a two-state solution in the Israel-Palestinian peace process before New Zealand's term is up at the end of the year and before Donald Trump's Government takes over in January.
Apart from the close working relationship on the Trans-Pacific Partnership trade deal, it is now other countries' problems and what New Zealand can do to help that defines the relationship with the US today.
Who knows when the next visit will be or by whom.
The bigger issue is whether the next administration will be as willing to pay New Zealand as much attention as the past one.