A 40-year-old police officer who is a father of six has been killed in a double shooting in Tamworth in northern New South Wales.

Police say highway patrolman Senior Constable David Rixon was shot several times outside of a block of flats at Coledale in Tamworth about 8:00am (AEDT) on Friday.

Senior Constable Rixon, who had over 20 years of experience in the force, was taken to Tamworth Hospital for treatment but died soon after.

Another man who was shot is under police guard in hospital undergoing emergency treatment.

A third man was arrested nearby and taken to Tamworth Police Station.

Joyce Glover saw the tragedy unfold.

"A police car went past and really fast, like nearly took the corner, and then next minute, bang, bang, bang," she said.

NSW Police Minister Mike Gallacher says Senior Constable Rixon was a much-liked policeman and dedicated family man.

"This is an incredibly tragic event for his loved ones, for his workmates and indeed for the state of New South Wales," he said.

Police Commissioner Andrew Scipione has arrived in Tamworth and says his prayers are with Senior Constable Rixon's wife Fiona and six children.

The Commissioner says police will use all available resources in their investigation.

"A number of firearms have been recovered but at this stage we're still piecing it together," he said.

"Every Highway Patrol in this state has in-car video and that's what we will be relying on and hopefully it will tell us a lot of what we need to know.

"At this stage, that has been secured and that's one of the first tasks, you can be sure, of the Homicide Investigation."

Commissioner Scipione says support is being offered to colleagues of the fallen officer.

"I understand that there were officers there, they were involved in CPR support," he said.

"You can imagine what that would have been like for those that serve alongside him every day.

"We'll offer every support. I can assure you that already we've dispatched a number of officers from our welfare services this morning. The Chaplain, I know is in the station."

'Day of mourning'

Senior Constable David Rixon was fatally shot after a routine vehicle stop. ( AAP/NSW Police/Northern Daily Leader )

The shooting comes one day after NSW Police celebrated 150 years on the job - a fact that was not lost on Mr Gallacher.

"As is the nature of policing, yesterday we were celebrating, today we are mourning the loss of a police officer," he said.

"Today we have an additional name to be remembered as a fallen police officer in NSW.

"It is a day of mourning not only for NSW Police but for the entire community.

"We cannot ever underestimate the impact these events will have on the community."

NSW Opposition Leader John Robertson said his thoughts were with the officer's family and colleagues.

"It's a sad day and a reminder for all of us just what a great job our police do and the risks they take every day," he said.

Off-duty colleagues of the policeman have arrived at the scene, visibly upset with their heads in their hands.

Police have launched an internal investigation into the shooting.