An assault charge on a man who came close to stabbing his partner's father has been dropped.

On September 26, Aaron Walter Zahnd was drinking with his partner's father when they started arguing, Whanganui District Court heard on Tuesday.

Zahnd went to a bedroom and returned with a 22cm hunting knife. He lunged at his partner's father, saying he would slit his throat.

His partner separated the two, and police were called. When Zahnd was searched another knife was found on his person.

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He was charged with threatening to kill, assault and possessing a knife in a public place.

He pleaded guilty to the threatening and possessing charges after Whanganui District Court Judge Dugald Matheson indicated the sentence could be six months' prison for the two charges combined.

At that police prosecutor Stephen Butler agreed to drop the assault charge, and Zahnd handed a letter expressing remorse to Judge Matheson.

The judge said Zahnd came very close to inflicting harm on his partner's father.

"Clearly in a domestic situation this man is a powder keg waiting to go off."

He ordered the destruction of both knives.

Zahnd's sentencing has yet to take place.