Williamson also decided not to accept a central contract last year, to leave himself free to play in Twenty20 and other leagues around the world. He instead signed a casual contract last year and still played in nine of New Zealand's 13 Tests in 2024, scoring more than 1,000 runs.
The master bat was absent from the list of 20 contracted players released by New Zealand Cricket on Tuesday, along with Devon Conway, Finn Allen, Tim Seifert and Lockie Ferguson who also are playing in T20 leagues overseas. He is expected again to sign a casual contract.
Allrounders Muhammad Abbas and Zak Foulkes, wicketkeeper Mitch Hay and spinner Adi Ashok are contracted for the first time in the absence of Tim Southee, who has retired, Ish Sodhi, Ajaz Patel and Josh Clarkson.
Williamson currently is playing for Middlesex in the English County Championship and T20 Blast and for the London Spirit in The Hundred. He is expected to have county commitments while New Zealand play Zimbabwe in late July and early August.
The 20 contracted players were ranked on domestic and international performances over the past year.
"The contracts with Mitch, Muhammad, Adi and Zak reflect the incredible talent coming through our system," New Zealand Cricket chief executive Scott Weenink said.
"These players have shown they can compete at the highest level and their hunger to represent the Black Caps is exciting. We're thrilled to see this group drive our team forward."
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