Police responded to the "valuable report" about people wearing masks while walking down Bishop St in St Albans about 10.45pm on Thursday.
"They were then seen entering a residential address further down the road, where they have forced entry and threatened the occupant with a weapon," said Christchurch metro area prevention manager Inspector Glenda Barnaby.
Two people were arrested, Barnaby said.
"Shortly after they gained entry, responding police located and arrested the two alleged offenders at the address.
"Luckily no one was injured."
Barnaby said a youth was due to appear in the Christchurch Youth Court in due course.
A 26-year-old man was set to appear in the Christchurch District Court today on charges of aggravated robbery and being disguised for burglary.
"This highlights the importance of reporting suspicious behaviour as soon as you see it, on 111," Barnaby said.
"In this case, it allowed police to respond to the robbery as it was happening and arrest those involved."
