Drinking throughout the night, Ashley Michael Archibald Casey got into an argument with his partner, became aggressive and slapped her so hard she fell on to a bed holding her jaw.
The 24-year-old pleaded guilty to assault through his lawyer James Elliott when he appeared in Masterton District Court beforeJudge Tony Walsh on Thursday.
About 5am on November 21, after a night of drinking and heavily intoxicated, he went into his girlfriend's bedroom and lay on the bed, police prosecutor sergeant Garry Wilson told the court.
"She declined and told him to sleep on the couch."
But instead of leaving the room, Casey became angry and the pair got into an argument, Mr Wilson said.
"He became aggressive and grabbed her by the wrist. She slapped him in the face in retaliation and he hit her back causing her to fall back onto the bed."
A friend at the house, whom Casey had been drinking with through the night, stepped in and intervened, he said.
When spoken to by police Casey said he was "in a bad mood due to his friend teasing him earlier in the night", Mr Wilson said.
Defence lawyer Mr Elliott said the couple were still in a relationship and were attending counselling together. Casey had a drinking problem, he said.
Judge Walsh said he was worried Casey had been before the court in July for assaulting the woman.
Casey needed to do something about his drinking, he said. "It's clear your actions were impaired with drinking too much alcohol ... your judgement was impaired. It was clearly a tense situation."
He warned Casey it could have ended more seriously as hitting someone in the head was potentially very dangerous.
"You caused her a lot of pain and discomfort ... hitting in the head is dangerous. You could have caused serious injury," he said.
Judge Walsh sentenced Casey to 80 hours' community work and nine months' supervision with orders to under take a stop violence and drug and alcohol programmes as ordered by his probation officer.
"There is to be no more violence. If you come back before the court on these charges again there will be more serious penalties."
