A young man who left explosives in a public place has been sentenced to community detention and supervision.

Tay Christopher Collins, who had no previous convictions, left an explosive in the Alexa Place walkway on St John's Hill in January. He pleaded guilty to the crime.

Collins, in his early twenties, appeared in the Whanganui District Court yesterday with his parents in court to support him.

Defence lawyer Roger Crowley said Collins was suffering from mental health issues, related to drugs, at the time.

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"This is a young man with some promise, a young man with some brains. Provided he can stay away from cannabis and live a lifestyle that ensures his mental health remains good, he will succeed."

Judge Jim Large said it was "an unusual piece of offending".

He said there were another two explosives that had been set off by Collins as well. When Collins' car was searched, a bottle of gunpowder was found.

"He said he was interested in making explosives," Judge Large said.

Collins told police he thought the area he was setting them off in would be safe.

The judge said Collins was "in the category of young men" who developed mental health issues and "probably psychosis" when they smoked cannabis.

He handed down a six-month community detention sentence with a 7pm to 7am curfew, as well as 12 months of supervision.

The young defendant also received a strike warning under the three strikes law.

Judge Large ordered the destruction of the explosive and gunpowder.