The 45-year-old suspected gunman, who was on parole, was arrested after being on the run for about an hour following the shootings in and around a hotel in Darwin's CBD on Tuesday.

He was arrested at the busy Stuart Highway and McMinn Street intersection near where the shootings occurred, with television footage showing him on the ground after being dragged from under his white dual cab ute, kicking his legs at officers from NT Police's Territory Response Group.

The incident started with reports of a man firing shots at Finnis St at about 5.50pm just outside the Darwin CBD, with the four deaths occurring at the nearby Buffalo Club, Gardens Hill Crescent, the Palms Motel and Jolly Street.

He also tried to enter the Peter McCauley Centre police station to possibly hand himself in, then contacted a duty superintendent who did a "magnificent job in negotiating with him" before the arrest, Police Commissioner Reece Kershaw told reporters.

The alleged shooter is an outlaw motorcycle gang member, well known to police with a criminal history, and the incident is not believed to be terror-related.

Mr Kershaw said he was was released from prison on parole in January after serving at least a year, was wearing an electronic monitoring bracelet, and had acted alone on Tuesday.

"He is an individual who is well known to police and has a number of interactions, adverse, with the police force. So he is well known to us," he said.

"We're still trying to establish the intent and the motivation behind this but all I can say is that, sadly, people have lost their lives this evening."

The man is being held in custody in hospital as a precaution.

Shortly after the shooting, a group of about a dozen men who were friends and relatives of one of the men who died gathered at the Palms Hotel, many in tears.

Local Matthew James helped patch up a woman who had been shot in the leg near the Palms.

She is in hospital in a stable condition, an NT Health spokeswoman said.

"She was caught up in it, she said 'I've been shot, I don't know why, I don't have anything to do with anything.' She was hysterical obviously," Mr James told AAP.

The woman who had been shot told Mr James the shooter had been going from room to room, shouting the name of a man he wanted to see and firing his gun.

"People were rushing past me saying someone had just been shooting up the Palms Hotel," he said.

One witness in the Pioneer Hotel across the road from the Palms initially believed he heard a firecracker.

"I would probably say that 15 to 20 rounds went off over the next five minutes and when we were sitting at the front here, it got very confusing, so everybody was taken inside," he told the ABC.

Another witness said he got one victim out of the motel.

"Me and this older gentlemen, we watched him walk from room to room shooting every room, and we couldn't go in there because it was a crime scene," he told the ABC.

"We got one person out, this gentleman over here but we didn't realise there was another person in there."

Speaking in London, Prime Minister Scott Morrison said the shooting was a "terrible act of violence".

"I just want to extend my deep condolences and sympathies to all the people in the Territory and particularly in Darwin," Mr Morrison said.

"This is a very tight community and I know they will be rocked by these events."

NT Chief Minister Michael Gunner said it was a devastating day for the Territory, "this was not the Darwin we know", and his thoughts were with families and friends of the victims.

Australian Associated Press