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However, SPS Security owner Danny Taimalie said the guard had earlier seen two of his workmates allegedly knocked unconscious by the man the guard later punched repeatedly and kicked on the ground.
The violence erupted about 1am on Stuart St outside Macs Brew Bar and resulted in five arrests plus charges for possession of methamphetamine and an offensive weapon.
Footage of the incident shows several people fighting on the road and one man pinning another to the ground.
A security guard wearing a hi-viz vest joins the fray and punches a man in the side of the head, continuing to throw punches along with another man, the footage shows.
The victim falls to the ground where the guard continues his attack, pauses for a moment, then appears to deliver one final kick.
Police arrived in a van shortly after and swarmed the scene, by which time the group of unruly revellers had become more subdued, and eventually arrested five people.
Mr Taimalie said two of his staff members were knocked unconscious by the man who was later captured on video footage being attacked by the guard, but was quick to say the guard's actions were unacceptable.
"That doesn't excuse his actions. He overstepped the mark."
He had spoken to the guard this morning and said disciplinary action would be forthcoming, though he said the guard's "aggressive'' conduct stemmed from seeing his two colleagues knocked down earlier.
"Those two guys have families, they're fathers.
"But I'm not trying to justify it, the force used was overly aggressive.''
The use of force by security should be the bare minimum needed to protect the guards or others, he said.
A police spokeswoman said a 33-year-old man is scheduled to appear in the Dunedin District Court on Tuesday charged with possession of methamphetamine and possession of offensive weapon.
A 62-year-old man is set to appear that same day, also charged with methamphetamine possession, the spokeswoman said.
The remaining three people received written warnings.
Police refused to be drawn on whether any of those arrested were security guards and said inquiries were ongoing.