“There’s so much T20 talent there and the next period will be important to get cricket into these guys and get them ready for the World Cup.
“Mitch [Santner] is a brilliant captain and leader – he has really come into his own with this team.
“It’s now their time to push the Black Caps forward in this format and I’ll be supporting from afar.”
The T20 World Cup in India and Sri Lanka starts in February.
Williamson made his T20 debut for New Zealand in 2011 but had not featured since June 2024.
Regarded by many as the best batter in New Zealand history, he is their leading Test run-scorer of all time and fourth on the ODI list.
New Zealand Cricket chief executive Scott Weenink said Williamson had earned the right to decide how he finished his ODI and Test careers.
“We’ve made it clear to Kane he has our full support as he reaches the back end of his illustrious career,” he said.
“We would, of course, love to see him play for as long as possible, but there’s no doubt whenever he does decide to finally call time, he will go down as a legend of New Zealand cricket.”
New Zealand completed a 3-0 clean sweep in the one-day series against England on Saturday, having lost a rain-affected T20 series 1-0 beforehand.