A shopkeeper felt extreme fear when three men entered his Black Bull liquor store carrying hammers and a steel bar, later discovering one was his friend.
The offenders, whose identities were partially concealed, confronted the victim and threatened him before stealing cigarettes, alcohol and cash from the store.
They made their escape from Duncan St just after 9pm in a vehicle driven by a fourth male, but were tracked down and arrested by police using dog handlers.
Shakaia Rapana-Tuirirangi, 17, was a friend of the victim. He pleaded guilty to aggravated assault and was sentenced by Judge Philip Crayton in Whanganui District Court.
Judge Crayton said this type of offending happens far too often in Whanganui and stores such as Black Bull were seen as relatively easy targets.