Oranga Tamariki says the video that has been circulating online of two teenage boys attacking one another at its youth justice unit was filmed by a staff member.
The incident took place on 16 June at its Auckland facility, Korowai Manaaki - the same place where five youths were on the roof at the weekend causing significant damage.
Four staff have been stood down pending investigations.
Oranga Tamariki deputy chief executive Tusha Penny told Morning Report the staff member's actions were "totally unacceptable and there are no excuses for it".
She said this was the only incident she was aware of that was videoed by a staff member.
A site manager came across the video online five days later and forwarded it to police.
Penny said Oranga Tamariki were dealing with "really challenging and complex teenagers" who have "committed very serious crimes in the community".
She said the training of the staff there was under review. Currently, they undergo training for "a couple of weeks" and are police vetted.
Police have confirmed they are investigating a fight at an Oranga Tamariki facility last month.
A spokesperson said no charges have been laid and the enquiries are ongoing.