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A Twitter user alerted Netsafe to the content on Friday morning, but two hours later the content was still online.
The content included an animation and what appeared to be grabs from the mosque shooting video, Netsafe's Martin Cocker said.
They had contacted Facebook and Twitter and the provider of the GIF content, which he identified as Tenor, owned by Google.
Police say they have also contacted the platforms "to ensure it is removed".
"We are undertaking enquiries into its origin," they told RNZ in a statement.
A Facebook spokesperson was not aware of the content when RNZ called them.
"As soon as we've become aware of this, we've been contacting our reporting channels" and the main online platforms, though this came to them from a third-party source, Cocker said.
"We'll be hoping to see it go down quickly, but a lot of that will depend on the trust and safety teams within that organisation."
The content might be illegal, though that would be up to the Chief Censor to judge, he said.
RNZ has contacted the censor's office.
Previously, police have charged dozens of people around the country over possession of the video of the mosque shooting.
"Derivatives of it are probably illegal," Cocker said.
The content was posted under a pseudonym but Netsafe wanted to know if the GIF provider knew anything about who created it, Cocker said.
"This is the kind of stuff that's hugely disappointing.
"To find something like this circulating and have to go through this process to remove something."
He was not aware of similar such content being posted before now.
Anyone who had copied the content should delete it, he said.