It's a bird, it's a plane, it's a… fuel tank.

It's probably not the adage you're familiar with, but it's not every day an aircraft fuel tank weighing up to 450 kilograms falls from the sky.

An aircraft was forced to dump the tank over Darwin on Monday during the first wave of flights for Pitch Black — one of the biggest international air force exercises in the Southern Hemisphere — after running into trouble after take-off.

"A jet aircraft involved in Exercise Pitch Black reported an engine malfunction and conducted an emergency jettison of its external fuel tank into scrubland south of Darwin," a Defence spokesperson said in a statement.

Darwin is hosting 140 aircraft and 4,000 personnel from 16 countries in the three-week exercise.

"It's certainly not a regular situation… the pilot suffered an emergency after take-off, unfortunately," Air Commodore Michael Kitcher, who is running Pitch Black, told ABC Radio Darwin.

"In dealing with that emergency in accordance with the… procedures, the pilot jettisoned the fuel tank and then safely landed back on the RAAF Base Darwin.

"I've been flying fighter aircraft for many years and this is a rare occurrence, I haven't seen this type of situation in Australia before."

Air Commodore Kitcher said the fuel tank landed in bushland without causing damage. ( ABC News: Jacqueline Breen )

Air Commodore Kitcher said investigations into the incident were ongoing, and could not confirm which aircraft was forced to eject the tank, or what country it belonged to.

"We've located the fuel tank, we know exactly where it is and there was no damage whatsoever," he said.

"It landed in bushland without causing any damage at all."

The aircraft and pilot returned safely to RAAF Base Darwin and NT Emergency Services were notified, a Defence spokesperson said.

The ADF and Northern Territory Police were at the site on Monday afternoon.

'It was tumbling over and over and over and over'

It was a strange sight for seasoned Darwinites, who have grown accustomed to the annual event.

"I was just watching this thing, and next thing I notice is that something actually fell off the jet," remarked Wayne Bell, who witnessed the incident from Darwin Kart Hire in Hidden Valley, around 10 kilometres from the CBD.

"I watched this thing fall and it was tumbling over and over and over and over.

"I was thinking, 'hang on, that's heading over that way somewhere, hopefully there's no one over there'."

Mr Bell, who previously served in the Royal Australian Air Force, said the plane did not appear to be Australian.

"[The tank] I'd say was half the length of the aircraft… at least six metres long I reckon, and probably nearly a metre diameter, a torpedo-shaped thing," he said.

"This thing just started tumbling down, down, down, down. I thought 'crikey, maybe there's going to be a cloud of dust or something over there'.

"I've seen a few things fall off aeroplanes, but nothing like that."

Exercise Pitch Black is the Royal Australian Air Force's largest and most complex exercise, operating out of Darwin. ( Supplied: Department of Defence )

In a Facebook post, Trey Branson said he was parked near the service station on Wishart Road when he saw "something pretty big" fall off a jet overhead.

He said it looked like it dropped "very, very close" to PowerWater Corporation workers in the power yard substation behind the petrol station.

The workers were "all staring towards where it landed … [it] just missed them," Mr Branson wrote.

"Felt it hit the ground. The two workers that were in the cherry picker in the air working must need to change their pants."

It's not the first time an aircraft has run into strife at Pitch Black; in 2000, investigators were left scratching their heads after a dummy missile fell from an FA18 Hornet onto a ute in Darwin.

Basil Rowe's ute was destroyed by the falling object, prompting calls for air warfare exercises to be moved away from the city.

"The bonnet's all ripped apart. The engine's just completely — it's in pieces everywhere, mate," Mr Roe told the ABC at the time.