Sydney: Four people were dead and one person injured in Australia after a gunman opened fire late on Tuesday in the northern city of Darwin, authorities said.

A 45-year-old man has been taken into custody and Northern Territory police say there are no further public threats. They said the incident was not believed to be terror-related.

"Today, 4 June, 2019, has been a devastating day in the Northern Territory," Northern Territory chief minister Michael Gunner told reporters. "Five crime scenes, four people deceased, one injured. This is not the Darwin we know," he added.

The alleged gunmen remained at large for an hour before he was apprehended.

Witnesses recalled harrowing scenes as a man moved from room to room opening fire at a motel.

The suspected shooter was known to police having been released from prison in January.

"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," Northern Territory commissioner Reece Kershaw told reporters.

Kershaw said they were investigating five crime scenes in the city.

The deceased were all male and the injured a women, he added.

Prime Minister Scott Morrison condemned the attack as "a terrible act of violence".

"I just want to extend my deep condolences and sympathies to all the people of the territory and particularly in Darwin," Morrison told reporters in London.

"This is a very tight-knit community and I know they will be rocked by these events." Witnesses told national broadcaster ABC they saw a man with a "sawn-off shotgun" enter a Darwin hotel.

"He shot up all the rooms, and he went to every room looking for somebody and he shot them all up, then we saw him rush out, jump into his Toyota pick-up, and rush off," witness John Rose told the ABC.

Another witness said she helped a bleeding women "with holes all in her skin" after her boyfriend carried the victim from the same hotel.