A Darwin man has described how he came face-to-face with a gunman and then ran, wearing only his underpants, and warned neighbours as his friend was killed.

Key points: John Reid has described running and hiding from the Darwin gunman while wearing only his underpants

John Reid has described running and hiding from the Darwin gunman while wearing only his underpants His friend and neighbour was killed at his home

His friend and neighbour was killed at his home He warned others on the street to hide indoors with their children

Four men in four locations across the city were killed by a lone gunman on Tuesday evening, while a woman was injured.

Police have a suspect in custody at the Royal Darwin Hospital.

One victim has been identified as Hassan Baydoun, 33.

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John Reid said he was rolling a cigarette outside his house on Jolly Street in Woolner when the gunman appeared.

"Thank to god I did not walk outside, I stayed where I was hidden," he told the ABC.

"It frightened the hell out of me, because I was kind of semi-naked, with my jocks on.

"I've taken off and I hid behind a couple of pods that we've got down the back, and I thought, 'bugger that', and I hid down the other side of the house."

He said as the gunman vanished one of his friends emerged.

"The next minute the other friend appeared out of nowhere, old mate, and he popped his head outside and he thought it was fireworks," he said.

"I turned around, 'it's not bloody fireworks, mate', I said, 'he's got a shotgun, bloke, get the hell out of here'.

"And he said, 'eh, shit that', and he jumped over the fence that quick.

"Now I'm a bit of a heavier guy, but I couldn't make it over the fence."

Mr Reid said he then saw the gunman come around the corner.

"He kind of looked at me, I looked at him, and he didn't raise the gun," he said.

"And then I looked at him again and then I just went 'see ya', and I was gone."

'I knew he was dead'

Mr Reid said he escaped under the fence of the property.

"By the time I made it underneath the fence I heard three shots and took off straight down the road, straight to the building down on the corner," he said.

"[A] family was outside. I told the lady outside, 'just get your kids inside, there's a guy down the road there with a bloody shotgun'."

Ambulance officers were seen outside one of the crime scenes in Woolner. ( AAP: Michael Franchi )

He said he hid behind a car, all the while wearing only his underpants.

"Then [my friend] came outside and said, 'here, put some shorts on, can't have you walking around with your jocks on'."

Mr Reid said the tactical response group from NT Police arrived not long afterwards.

"They kind of frightened me because they had bloody machine guns, and I was just about scared of guns at that time, after seeing the gunshots happening over there at the bloody house," he said.

"I said, 'somebody go in there and check the house because my mate's inside there and he's dead'. I knew he was dead."

He said he'd spent a sleepless night at a friend's house.

"I tried to go to sleep, and it's not funny when you hear gunshots like that. It frightens the hell out of you," Mr Reid said.

When asked if he'd feared for his own life, Mr Reid said: "I'll give you three guesses, but you're only going to need one.