The World Cup-winning rugby player tweeted the graphic images of two dead children after returning from a trip to Syrian refugee camps in Lebanon with the charity.
He included a message to his more than 500,000 followers:
"What did these children do to deserve this? This summer share a thought for the innocent lives lost everyday in war."
The high-profile sportsman travelled to camps in the Bekaa Valley earlier this month as a guest of UNICEF New Zealand. The experience clearly had an impact on him.
A representative of the charity in Australia, Kate Moore, says UNICEF does not support the sharing of such images.
“I completely understand his outrage. He spoke to children. He understands the atrocities that they’ve seen. The picture he shared is a reality of what goes on in conflict zones, not just in Syria but around the world.
However, we would never put into the public domain an image like that. The dignity and rights of the child disappear when we start sharing a picture like that. UNICEF would aim to protect the dignity and rights of the child.
The images also drew a mixed reaction from Williams' Twitter followers.
"Although I sympathise w ur reasons for posting this, it's not wise. It can be a trigger for some pple. Also young followers," one said.
"Insensitive. He's a rugby player with young followers who follow him because he's their sports hero, not for politics," tweeted another.
Others tweeted messages of support, such as:
"It is a ferocious reality. Good on him for sharing. People can un-follow."
'There are other ways to tell those stories'
Executive Director of the charity in New Zealand, Vivien Maidaborn told SBS the charity cannot condone the sharing of graphic images of children.
However, Ms Maidaborn says she understands why Williams felt compelled to do so.
"I completely understand the process he's going through," she said. "It's a very humane response to the horror of war [and[ connecting up with that knowledge about what's happening in Syria for the first time and being surprised that more people aren't horrified and shocked."
Williams' graphic tweet has been shared more than 1,400 times. Meanwhile videos, such as those telling the stories of children Williams' met, have only been viewed a few hundred times.
"Telling that story of a child who is striving right now to put her life back into something that makes sense, while staying hopeful of the future is a very powerful way to engage people," Ms Maidaborn said.
"And we may not engage the people who are just responding to shock and horror - in our experience, those people don't often follow through with a practical action that helps us help those children."
“In the end that’s what we’re counting. We're not counting the number of tweets or retweets. We're counting the amount of conversion of interest to support for our work.”
