They had contacted Facebook and Twitter and the provider of the GIF content, which he identified as Tenor, owned by Google.
Police said they also contacted the platforms "to ensure it is removed".
"We are undertaking inquiries into its origin," they told RNZ in a statement.
The Chief Censor and Classification Office said the footage content was "a real concern" and "harmful", particularly for attack survivors, and people should not share it.
A Facebook spokesperson was not aware of the content when RNZ called the company.
Google accepted something went wrong in allowing the offensive material on one of its online platforms.
"Terrorist content has no place on Google-hosted platforms, including Tenor," a spokesperson said.
"This content violates our policies and we worked quickly to remove it."
According to Google, the content could still be available for some users until the cache was fully updated.
The company accepted no system was perfect and something went wrong on this occasion. It said it would review its approach.
People in online posts commented that it was "atrocious".
"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, Cocker 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.
People can report such content to Internal Affairs or Netsafe.
- RNZ
