Wadsworth, 18, was arguing with his partner while they were walking along the rail bridge.
The argument was intense enough that a passer-by with his four-year-old son asked Wadsworth to stop it, but it only set him off worse.
Wadsworth turned his attention to the victim, abusing him verbally and threatening him with violence.
He received a small metal chain from his partner and began waving it threateningly towards the victim, who backed away off the bridge.
Wadsworth appeared for sentencing before Judge Philip Crayton in Whanganui District Court on Tuesday.
He pleaded guilty to threatening to kill or do grievous bodily harm, possessing an offensive weapon and other miscellaneous threats and intimidation.
"I have a victim impact statement in relation to the sixth of October which identifies that the complainant has suffered emotional harm," Judge Crayton said.
"He expresses the fear of being intimidated when he was out with his son and the effect also on his son."
The judge sentenced Wadsworth to 80 hours' community work and 12 months' supervision.
"You know that if you're ever before the court again for matters like this, you'll be on for either a more stern sentence or possibly imprisonment," he said.
"People have the right to not be affected by threats and by your behaviour."