A man who broke into his ex partner's house said he did it because he was worried about her mental health.

Micah Conan Robertson went to the victim's Whanganui house on March 29, Judge David Cameron said in the Whanganui District Court on Tuesday.

Robertson was "fearing for her mental state and her safety", Judge Cameron said.

"That's his position anyway. The police position is that when there, he was huffing on a butane canister and went around to the back of the house as an associate of the victim came to the front door.

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"This defendant smashed a single pane of glass in the bathroom window."

In a victim impact statement, the ex partner said she was "terrified" to hear Robertson had broken into her house.

He pleaded guilty in court to wilful damage and received a one week prison sentence, and was ordered to pay $60.68 reparation.

In an unrelated incident, Robertson was spotted on the corner of Ingestre and Purnell Streets on May 4 "holding and waving a knife around, looking intoxicated", police prosecutor Sergeant Rachel Willemsen said.

Police were called and found Robertson was under the influence of butane gas, from several gas cans he was carrying.

The area has a high amount of foot and road traffic at the time he was there, and is close to a school.

He pleaded guilty to possessing a knife in a public place, and Judge Cameron sentenced him to another week in prison.