Duty Superintendent Lee Morgan told CNN that a man had been taken into custody at approximately 7:45 p.m. (6:15 a.m. ET), about an hour after the shooting -- which took place in Darwin's inner suburbs -- was reported to police.
"The situation is now under control," Morgan said, adding there was no "ongoing threat to the public."
At a press conference on Tuesday evening, Police Commissioner Reece Kershaw said the suspect was an individual who was "well-known to police," who had been on parole following his release from prison in January.
Kershaw said that police were still trying to establish a motive for the shooting and that the investigation was ongoing, adding: "At this stage, we do not believe it is terrorism-related."
The shooter is believed to have fired multiple shots with a shotgun before he made contact with police, who said he might have been trying to hand himself in.
Australia's Northern Territory Chief Minister, Michael Gunner, said there were "five crime scenes, four people deceased, one injured" and described the incident as "a devastating day for the Northern Territory."
In a message on Twitter, Australia's Prime Minister Scott Morrison called the event a "terrible act of violence."
Shootings are rare in Australia, which introduced tight gun control measures after a 1996 massacre at Port Arthur, a popular tourist spot in Tasmania. Then, 35 people were shot dead by a lone gunman armed with a military-style semiautomatic rifle.
John Howard, Australia's then-prime minister, banned rapid-fire rifles and shotguns and tightened licensing regulations after the massacre. More than a million firearms were collected and destroyed.
Last month, a security guard was killed and five others were wounded in an apparent drive-by shooting incident outside a Melbourne nightclub.