Constable Soames Crowther sprung into action when he saw a man running down the street with a cash box and a knife.
A man with a knife allegedly threatened the attendant at a dairy on Wainoni Rd in Avondale, Christchurch about 12.30pm today.
The shop attendant was not injured but the offender allegedly took off with the shop's cash box.
Phillipstown neighbourhood patrol Constable Soames Crowther was driving past at the time and saw people watching a man with a cash box and knife running away.
He jumped out of the car, chased the man and arrested him without drama.
"It's the first time I have caught someone so quickly, but two passing members of the public were instrumental in assisting me in the arrest and I am very grateful for their help," he said.
Area Prevention Inspector Manager Richard Bruce said police would not tolerate threats of that nature towards members of the community.
"Based on the good information provided to us by the public and a quick police and public response, this job got solved instantly and no one got hurt, which is very lucky. We are grateful to those members of the public who helped our constable," he said.
"This is another arrest which is significant for providing reassurance to the victims and preventing further serious incidents of this nature.".
A 15 year-old male was arrested and will appear in the District Youth Court this week facing charges of aggravated robbery.