Four people are dead and two injured in Australia after a gunman opened fire late Tuesday in the city centre of Darwin, police said.

A 45-year-old man has been taken into custody and police say there are no further public threats.

"The active shooter threat has dissipated," superintendent Gavin Kennedy, from the Northern Territory police, told AFP.

Kennedy said police were investigating at least five crime scenes in the city.

Police had earlier shut down parts of the city and alerted the public an armed offender was on the loose.

Prime Minister Scott Morrison said the attack was not believed to be terror related.

"Our advice is that this is not a terrorist act," Morrison told reporters in London.

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 a boyfriend carried the victim from the same hotel.