The 22-cap player sparked questions when he failed to show up to the training session overnight ahead of the team's game against Wales in Cardiff, but his absence was later explained by a tweet.
"The proudest moment of my life," Williams posted on Twitter, alongside a photograph of his newborn daughter's tiny foot.
It is understood Williams followed his wife Alana's progress from the team's Cardiff hotel over Skype.
He became the second All Black to celebrate the birth of a child in as many weeks, with prop forward Charlie Faumuina confirming his second child was born last week.
The proudest moment of my life #alhumdulliah pic.twitter.com/laO21VLAj9
— Sonny Bill Williams (@SonnyBWilliams) November 18, 2014
"We are delighted that Sonny and his wife Alana have had a little baby girl," said assistant All Blacks coach Ian Foster.
"He was following that event so we are delighted that the family is all good. And on top of Charlie and Rochelle's second child last week, it has been a week of celebration for the All Blacks with babies.
"Sonny wasn't at training simply because he was monitoring the progress of his wife who did an outstanding job by all accounts."
However, Williams will not fly home to New Zealand to be with his new family until after the test against Wales this weekend. Faumuina will also not be going home before the final test of the year.
Williams' cute tweet has already been favourited by fans more than 1300 times and retweeted more than 650 times. The All Black has been inundated with messages from fans sending their congratulations to the couple, one telling him: "Welcome to the best club in the world."
Black Fern Honey Hireme tweeted her well wishes, saying: "Congratulations Sonny to you & your family! Welcome to parenthood, life changing & by far the best gift in life."
The All Blacks Twitter account also congratulated its players, posting: "Congrats to new Dad @SonnyBWilliams & wife Alana on birth of baby girl & @BIGGrizzly3 & wife Rachelle who had 2nd child, a girl, on Friday."
- Gregor Paul of the Herald on Sunday and Patrice Dougan of NZME. News Service
