Mike Rutene John Pinnock stood silently in the dock at Whanganui District Court as Judge Gregory Ellis demanded to know what gave him the right to "push an old man around".
He appeared in court for sentencing on the charge of common assault, to which he pleaded guilty.
Pinnock pushed his partner's father in June during an argument, police prosecutor Sergeant Drew Morrison said on Tuesday.
The victim fell and stayed on the floor for five minutes "unable to get up" due to a medical condition, Mr Morrison said.
Defence lawyer Anna Brosnahan said Pinnock was frustrated because the victim had been coming into the house without permission.
"He had asked the victim to leave, hence the push towards the door," Ms Brosnahan said.
She said the victim was entering a house that he wasn't a resident at with a key that he shouldn't have had.
Pinnock was on a community detention sentence at the time and could not leave the address.
Judge Ellis noted the detention sentence was as a result of a conviction for injuring with intent to injure.
"You may be her partner but that gives you no right to lay your hands on him," said Judge Ellis, sentencing Pinnock to 80 hours of community work.