There is camera footage from inside one of the mosques which shows survivors walking around among the bodies. If that's from the second mosque, then it's possible someone was recording while the attack was in progress.
There is also footage recorded by a bystander outside the first mosque.
Of course whoever recorded these probably broke the law because the NZ censor is saying "objectionable" footage is illegal at the moment it is created and not when officially declared so by them. Given how often footage shot by survivors or bystanders helps investigators, this is a foolish attitude to take.