Don't be so sure that is what happened. In the commentary with the footage at the following link, the 7 News presenter says: "..been obtained exclusively by 7 News... the vision obviously will be passed on to the experts investigating this tragedy". So sounds to me like 7 managed to get hold of the material first, then planned to pass to ATSB. I can well imagine media people managed to make it over to the crash site long before any ATSB personnel, or perhaps even police, arrived on the scene. Passenger may have handed over material under a false impression they were giving it to an official investigator. You can bet this will be investigated as well now.
That is the more likely scenario. Any material that has been formally obtained by the ATSB cannot be released to the public. This footage may not even be required by the ATSB as they may have more than enough. Have you noticed that there is no footage from the other helicopter? I would imagine that is of more interest to the investigators. Even the surviving pilot is not prevented by the ATSB from telling the media his story. The only thing he can't do is provide the media with a transcript or an audio copy of his discussion with the ATSB. If someone was trying to impersonate an official investigator the ATSB would have to refer any investigation into that matter to the AFP. The investigation would only proceed if the AFP thought that by impersonating a Commonwealth Officer (thats what investigators are) that person was able to obtain some form of financial reward from the Commonwealth or hindered the investigation. Anyone can conduct their own investigation of this event, as evidenced by this thread. The difference with the ATSB is that they have the legal backing to obtain whatever they need to conduct the investigation and that evidence is not available for public scrutiny even for FOI requests.
Realistically the ATSB already know what happened. Two modern helicopters collided on a clear day conducting joyflights. Its the why that will be the focus of the investigation particularly why a helipad with a white cross on it was being used. The fact that the CP was flying from that helipad would be significant as he would be the only pilot within the organisation that could authorise its use. The focus will be on the whole organisation ,including Sea World management, and its contribution to the accident. The surviving pilot was unfortunately the last line of defense but if he wasn't told that the old helipad was being used then why would he look in that direction.
To all those who shout " He should have had a better scan" or " He was looking in the wrong spot" consider this. How many of you look behind when proceeding at a stop or give way sign?