He became angry at her and punched her head several times. The woman ran to the kitchen where she got a large knife and returned to the bedroom to confront Manihera.
Manihera closed the door and the pair yelled abuse at each other until police arrived.
The woman ended up with two large bumps on her forehead and was taken to hospital by ambulance for observation.
Manihera told police he had acted in self defence.
Defence lawyer Virginia Pearson said the knife brandished by the woman had been confiscated but she was not charged.
"This defendant has ended up with injury himself, he's pleaded on the basis that the amount of punches have been an overreaction to the situation."
She said there was a history of problems between the couple, who have been in a casual relationship for the last three years, and that Manihera wanted nothing more to do with the woman.
"This is the end score for him, he's no longer wishing to have any contact with this person."
Judge Chris Tuohy said the fact the woman had not been charged was not a reason for not charging Manihera.
"It may well be that it started because you got hit over the head with a laptop and later got threatened with a knife but two wrongs don't make a right and what you did was wrong."
Judge Tuohy sentenced Manihera to 75 hours' community work.
Ms Lawrence told the Times-Age it was "unlikely" the woman would be charged.