But Mr Newdick says the constable is lying and became volatile when he asserted there were no grounds to prevent him leaving a cordoned area.
He had been working for 21 hours and wanted to get home to sleep.
The constable punched him with a closed fist three times inside his vehicle, handcuffed him by one of his wrists and dragged him out of the car on to the road surface, where he cuffed both his hands then kicked him in the head as he lay on the ground, Mr Newdick alleges.
Throughout the assault, the constable verbally abused him and said he should have complied, the court heard.
The assault left him close to unconscious, with blood pouring out of a wound to the corner of his eye, Mr Newdick said in evidence.
He was taken to the police station and charged with resisting and obstructing police - charges that were later withdrawn.
His memory had been affected by the alleged attack and he now found it difficult to recall some of the occurrences that night and other past events, Mr Newdick said.
During cross-examination, defence counsel Doug Rishworth said Mr Newdick had an anti-authority attitude and a particular disdain for the police.
He had at one point made an inquiry to the police about the diversion programme, saying he would plead guilty to the resisting and obstruction charges in exchange for diversion.
He had a propensity for obstructing police as demonstrated last year when he was stopped while driving and questioned about a domestic incident. On that occasion he had been unco-operative, had challenged being stopped, had got out of his vehicle and waved his arms around - just as he had at the cordon.
Mr Newdick was hysterical, emotional and had overdramatised the whole thing, Mr Rishworth submitted.
Mr Newdick had behaved at the cordon as if he was drunk, yet was not.
Later, at the police station, he had whimpered and cried, walked with a limp and collapsed on the floor.
Mr Newdick denied his actions were exaggerated, saying his behaviour had been due to his ordeal.
"I didn't do the damage to myself, if that's what you're implying," he said.
He accepted Mr Rishworth's assertion that he had punched a wall while later laying his complaint at the police station.
Senior Sergeant Grant Pigott, who organised the cordon, said in evidence he went to check on the constable who had reported having difficulty with a car. He was conscious that due to staffing constraints, the constable was working alone.
"Newdick was very worked up and excitable - crying almost and hyperventilating. The constable was as lucid and calm as can be, using colourful language and swearing. It was obvious there'd been a conflict but he wasn't out of control. He was still composed. I had no concerns about him."
The constable told him the complainant's injuries were caused by the road surface.
"I checked (the constable's) hands and clothing. I noted nothing alarming to me," Sergeant Pigott said.
A witness who lived near the scene said he heard the constable repeatedly swearing at the complainant and telling him to get out of the vehicle.
The witness was able to see the incident because it occurred inside the complainant's vehicle. The constable appeared to yank him out of the vehicle. The complainant was holding the steering wheel, screaming "I'm sorry, I'm sorry".
He did not have a full view of the incident when it continued outside the car but, from what he saw, it looked as if the constable's arm was "going up and down on him... as if he was hitting something", the witness said. The complainant was screaming for help.
