A novice base jumper has narrowly avoided leaping to his death after an eagle-eyed instructor spotted he had failed to fasten his harness correctly.

Professional jumper Chris McDougall, from Australia, had been teaching students on a 110m high bridge in Croatia when a jumper from outside the group climbed over the railings without putting on his leg straps.

Mr McDougall noticed the mistake and launched a expletive-filled warning at the man who was just moments from jumping.

The novice jumper (Chris McDougall/ YouTube)

In a video of the incident uploaded to YouTube on Friday, Mr McDougall can be heard shouting "Stop! Stop! Stop!" after the man dropped his parachute over the side of the bridge.

"This random jumper came rushing out [...] and had clearly no idea what he was doing," said Mr McDougall following the incident.

"This guy just throws over his gear [...] he climbs over the railing [...] and then I notice he's got no leg straps on.

"I caught him within about one and a half seconds of him jumping straight to his death.

"If he'd have jumped off that object he was going to die."

Professional basejumper Chris McDougall (Chris McDougall/ YouTube)

The incident did not stop the thrill-seeker from base jumping however, and he attempted the jump again after he had fitted his equipment correctly and landed safely.

BASE jumping - which stands for buildings, antennas, spans and earth, from which jumps are performed - is one of the world's most dangerous sports. Unlike a parachute jump from a plane, BASE jumps are done from fixed objects at lower altitudes.

A list maintained by Blinc has recorded 299 base jumping deaths since 1981.

“[I] hope this video will raise awareness to how important it is to pay attention to detail when base jumping," said Mr McDougall.

"So many new jumpers come into this sport thinking it is all about GoPro's and social media and they forget the fundamentals of how serious and dangerous this sport can be if respect is not given to every aspect of it.