In NZ it is illegal to post the video or the manifesto made by the shooter on the internet (no First Amendment in NZ). Someone has already been arrested for it. The penalty is a max 10 years in prison.
https://abcnews.go.com/ABCNews/zeala...l_twitter_abcn
The email to the forum owner was a request to retain data so that a warrant could be issued to determine who in NZ had posted links to the video and/or the shooter's manifesto on his forum.