New Zealand made 315 for seven in their 50 overs and had the tourists at 166 for six when rain stopped play in the 31st over.
Pakistan had arrived on a nine-match winning streak but fresh from playing in more accommodating conditions than the 120 kilometre an hour (75 mph) winds and rain that greeted them in Wellington. After being sent in to bat, Colin Munro (58) and Martin Guptill (48) gave New Zealand a flying start with 83 for the first wicket before Williamson went to the middle with the dismissal of Munro in the 13th over.
While the openers plundered the boundaries, Williamson's 115 off 117 deliveries came from a diet of ones and twos with only eight fours and one six. "It was holding in the wicket a little bit, and you come to a point in your innings where you either address it sensibly and accept that's what it is doing, or you do something silly," Williamson said.
"Today I was a little bit more sensible and accepting of the fact that they did bowl well for a long time there, and I felt we were perhaps fortunate to get that above 300 score. "They did execute their plans well, the wind was tough to deal with, and maybe that's where we gained an upper hand, but you do ebb and flow through an innings."
Williamson, who was dropped on 26 by Pakistan wicketkeeper and captain Sarfraz Ahmed, also featured in a 90-run partnership off 80 balls with Henry Nicholls before he was caught by Hasan Ali off Rumman Raees in the 48th over.
Ali was central in most of the key New Zealand wickets with the dismissals of Munroe, Nicholls (50) and Ross Taylor (12) to finish with three for 61. Pakistan were in trouble in the very first over of their reply when Tim Southee took the wickets of Azhar Ali and Babar Azam, both lbw. Fakhar Zaman battled bravely to try to restore the Pakistan innings and was unbeaten on 82, the only innings of note, when rain stopped play and Southee had the figures of three for 22. "It's a setback for us, especially after losing two wickets in the first over," said Sarfraz who also shouldered part of the blame for his own fielding lapse. "The New Zealand's batsmen batted well, especially Kane Williamson. If you drop catches, it becomes tough. Hopefully we will sit together and come up with a better performance next time."
The second match in the five-match series is in Nelson on Tuesday.
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Scoreboard New Zealand
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M. Guptill c S. Malik b F. Zaman 48
C. Munro c S. Ahmed b H. Ali 58
K. Williamson c H. Ali b R. Raees 115
R. Taylor b H. Ali 12
T. Latham c B. Azam b F. Ashraf 3
H. Nicholls c S. Ahmed b H. Ali 50
M. Santner c R. Raees b M. Amir 7
T. Southee not out 12
T. Astle not out 0
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Extras: (lb3, w7) 10
Total: (for 7 wickets; 50 overs) 315
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Fall of wickets: 1-83 (Munro), 2-156 (Guptill), 3-192 (Taylor), 4-198 (Latham), 5-288 (Williamson), 6-297 (Nicholls), 7-311 (Santner)
Bowling: Mohammad Amir 9-2-57-1 (1w), Rumman Raees 10-0-68-1 (2w), Hasan Ali 10-0-61-3, Shadab Khan 9-0-49-0 (1w), Faheem Ashraf 9-0-58-1 (2w), Fakhar Zaman 3-0-19-1
Pakistan
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Azhar Ali lbw b Southee 6
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Fakhar Zaman not out 82
Babar Azam lbw b Southee 0
Mohammad Hafeez c Southee b Boult 1
Shoaib Malik c Latham b Southee 13
Sarfraz Ahmed lbw b Astle 8
Shadab Khan c Latham b Boult 28
Faheem Ashraf not out 17
Extras: (lb1, w10) 11
Total: (for 6 wkts; 30.1 overs) 166
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Did not bat: Mohammad Amir, Hasan Ali, Rumman Raees Fall of wickets: 1-6 (Azhar Ali), 2-7 (Babar Azam), 3-13 (Mohammad Hafeez), 4-37 (Shoaib Malik), 5-54 (Sarfraz Ahmed), 6-132 (Shadab Khan)
Bowling: Southee 6.1-0-22-3 (2w), Boult 6-1-35-2 (2w), Ferguson 7-0-40-0 (3w), Santner 6-1-39-0 (3w), Astle 5-0-29-1.
Toss: Pakistan
Result: New Zealand won by 61 runs under Duckworth-Lewis system
Umpires: Wayne Knights (NZL), Ruchira Palliyaguruge (SRI)
TV umpire: Ray Illingworth (ENG) Match referee: Richie Richardson (WIS).