In December 2015, Nicholls was named in New Zealand's
One Day International (ODI) squad for their
series against Sri Lanka. He made his ODI debut on 26 December 2015 at
Hagley Oval,
Christchurch, which is his home ground for Canterbury. He scored 23 not out off 21 balls in that game as New Zealand won by 7 wickets. He scored 82 runs,
against Pakistan on 25 January 2016 at
Basin Reserve, which became a match-winning knock at the end. New Zealand won the match by 70 runs and Nicholls was adjudged man of the match. He made his
Test debut on 12 February 2016
against Australia. In February 2016, Nicholls was added to New Zealand's squad for the
2016 ICC World Twenty20 tournament, also as a backup wicketkeeper to
Luke Ronchi. He made his
Twenty20 International debut for New Zealand on 26 March 2016 against Bangladesh in the
2016 ICC World Twenty20 tournament. In March 2017, during the second Test
against South Africa at the
Basin Reserve, Wellington, Nicholls scored his first century in Tests, scoring 118 in the first innings. In May 2018, he was one of twenty players to be awarded a new contract for the 2018–19 season by
New Zealand Cricket. In January 2019, during the third ODI
against Sri Lanka, Nicholls scored his first ODI century, making 124
not out off 80 balls. In April 2019, he was named in
New Zealand's squad for the
2019 Cricket World Cup. In the final, he top scored for New Zealand with 55 runs. In November 2020, Nicholls was named in the
New Zealand A cricket team for practice matches against the
touring West Indies team. In November 2023, Nicholls was reported to New Zealand Cricket for an issue surrounding
ball tampering during a Plunket Shield fixture between Auckland and Canterbury. In the 32nd, 35th and 37th over Nicholls was caught rubbing the ball against his helmet. During the second ODI
against Bangladesh, on 20 December 2023, he scored 95 runs and completed his 2000 runs in ODIs, Nicholls was included in the ODI and T20I squads for the
Black Caps' 2024 tour of Sri Lanka, but only featured in the ODIs. == Personal life ==