He said his grandson's mother was "startled" at today's events.
The teen was born in Taumarunui but was living with his grandparents at Raurimu since December, after living on a dairy farm near Taupo.
The grandfather said his grandson owned a replica AK47, which shot BB pellets, and he hoped that was the weapon he was using today rather than a real weapon.
But he said his grandson would be facing a lot of questions.
"He's been through a pretty bad life...so we were trying to put him on the straight and narrow."
Today's troubles may have stemmed from a difficult upbringing, he said.
Police had not done a good job in communicating with him about the charges he faced, the grandfather said.
Mr Mastrovich said officers had secured six crime scenes in "a variety of locations" and were interviewing a range of witnesses to piece together the teenager's movements.
"We believe the man has committed several serious offences in a number of different locations, including discharging a firearm in an unsafe manner," Mr Mastrovich said.
Jason Dickinson was staying with his wife and baby daughter at his parents' place on Miro St when the teenager came to the door asking to use the phone to call a car yard because he had crashed nearby.
Mr Dickinson said he seemed polite but once inside he allegedly pulled a knife on his father, Rod Dickinson, and stole one of the cars parked at the house.
The shocked family watched the offender drive away and called police while at the front of the home.
Mr Dickinson said he was believed to have stolen fuel from a nearby petrol station and then driven back past the family house.
"The guy went back past with my car that he'd stolen and he stopped up the road and let a bullet off," Mr Dickinson said.
"He stopped and pointed it at us and told us he would blow our f****** heads off.
"It was pretty crazy s***.
"He slowed right down and jumped out of the vehicle and pulled out the rifle and let a shot off ... and then he took off back into town."
Mr Dickinson said his family was shaken up by the incident.
"I'm just happy my wife and daughter are all right," he said.
There were earlier reports that three people were shot but an ambulance spokesman said paramedics called to the scene were stood down without being needed.
The armed offenders squad had cordoned off areas of the town while the teen was being pursued.
Local woman Gemma Carlyle said the Central Park Motor Inn was forced into lockdown by police patrolling the area.
"I was scared. There were [a lot] of police and then there were the dogs and everything," she said.
"We had to stay inside and close all the doors."
Mr Mastrovich said cordons were later lifted and police were not looking for anyone else in relation to the offending.
The arrested man was in custody and "will appear before the courts shortly", Mr Mastrovich said.
Police have appealed for witnesses to contact officers.
Anyone with information should call Taumarunui Police on 07 895 8119 or Crimestoppers on 0800 555 111.
