Ross Taylor posted his 20th one-day international century and Henry Nicholls scored his first as New Zealand beat Sri Lanka by 115 runs to sweep the ODI series 3-0.
Taylor made 137 and Nicholls 124 not out as New Zealand reached 364-4 batting first.
Sri Lanka were all out for 249 in 41.4 overs in reply, threatening New Zealand's total briefly, especially when Thisara Perera (80) was at the crease, before again falling short.
Paceman Lockie Ferguson took 4-40 and leg-spinner Ish Sodhi snared 3-40.
Taylor became the first New Zealander to score 20 centuries in a single international cricket format and continued an extraordinary run of form which has seen him score 181 not out, 80, 86 not out, 54, 90 and 137 in his last six ODI innings.
New Zealand were 31-2 when Taylor went to the crease but he built partnerships of 116 with Williamson (55) and 154 with Nicholls to lead his team to a 300-plus total for the third time in the series, the hosts scoring 69 from the last five overs.
Sri Lanka were on pace early in the run chase but then slipped to 143-5 in the 23rd over of the innings, while they then tumbled from 244-5 to 249 all out once Thisara was out to a superb catch from Martin Guptill.
Watch the one-off Twenty20 international between New Zealand and Sri Lanka, live on Sky Sports Cricket (channel 404) from 5.55am on Friday.