Before assaulting his partner, Isaiah Chas Kemp took her wallet, snapped her bank cards, and threw her personal belongings out a car window.

Kemp was in a car with the victim, being driven by his grandmother, when he offended.

He started by taking her bag and grabbing her wallet, police prosecutor Sergeant Stephen Butler said in the Whanganui District Court on Tuesday.

Kemp also went through the victim's bag, looking for her cellphone.

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When Kemp's grandmother pulled the car over, Kemp tried to force the victim to get out of the car, grabbing her by the jacket.

He couldn't pull her out though, Mr Butler said.

Kemp kicked the victim in the leg, and his grandmother intervened.

When spoken to by police he said "he had been unable to forgive a past indiscretion", Mr Butler said.

Kemp pleaded guilty to common assault. Defence lawyer Anna Brosnahan said the relationship was over.

"It probably didn't end in the most dignified way, sir," she said.

Judge David Cameron sentenced Kemp to 75 hours of community work.

He was also given six months of supervision.