A "light-hearted" tussle with another reveller quickly got out of control, the court heard.
Ratapu first lashed out at a 32-year-old woman who tried to calm him down, tackling her to the floor and punching her in the face.
Once he left her bleeding from her nose and ear, he said he would knock out anyone else who stood in his way.
A younger woman, who also tried to de-escalate the situation, was punched in the face, grabbed around the throat and shoved into a wall.
Ratapu then turned his rage on his partner, grabbing her by the head and pushing her into a door frame.
He punched a fourth woman who stepped in before striking his girlfriend a couple more times.
After heading out of the house, Ratapu swung and missed at a man then punched his partner in the jaw, knocking her out.
The defendant was similarly wild in his dealings with police, attempting to tackle an officer as he was being handcuffed.
Six months earlier, the court heard, Ratapu had a similar run-in with police because of his aggressive conduct.
They arrived after the defendant punched his girlfriend's brother in the chest, and found him in an "extremely agitated and heightened" state.
When Ratapu disobeyed instructions, he was pepper-sprayed and it took three officers to subdue him.
Scally said her client had been diagnosed with post traumatic stress disorder after an incident in which he sustained severe head trauma.
That, combined with alcohol use disorder and cannabis dependency, would likely have contributed to the "gross" violence, Judge Emma Smith accepted.
Ratapu's remorse, she believed, was genuine.
Since being behind bars on remand, the defendant had completed a variety of programmes, which the judge said spoke to his commitment to rehabilitation.
Because of his issues, he would require "comprehensive work" in the future.
Judge Smith granted leave for Ratapu to apply to have the prison term converted to home detention, if a suitable address became available.
