Prime Minister Scott Morrison said the incident was not terrorism related. "This is a terrible act of violence that has already, I'm advised, taken the lives of four people," Morrison told reporters in London.
A 45-year-old man was in custody following the shooting, Northern Territory Police Duty Superintendent Lee Morgan told Guardian Australia.
"At this stage, we've got reports of four deceased and a number of other people who have been shot," Morgan said.
Police contacted by The Associated Press declined to comment.
A man fired a pump action shotgun at the Palms Hotel in the Darwin suburb of Woolner in the late afternoon, Australian Broadcasting Corp. reported. Police attended three crime scenes in the city of 100,000 related to the gunman, ABC said.
Police earlier said the suspect was described as wearing a high-visibility shirt and driving a white dual-cab pickup truck, Australian Associated Press reported.