A giant NASA science balloon crashed during take-off in Australia Thursday, destroying its multi-million-dollar payload, toppling a large car and narrowly missing frightened observers.

Dramatic footage of the incident showed the balloon's large undercarriage coming loose from its moorings, smashing through a fence and knocking a four-wheel-drive car on its side before coming to rest.

"We were sitting in our car and preparing to move it out of the way and we actually were within a foot (30 centimetres) of being wiped out," a relieved bystander said, on footage relayed by public broadcaster ABC.

"If it hadn't been for the other gentleman's car being there, we'd be somewhere else by now, I think."

The balloon, the size of a football field when inflated and designed to float up to 40 kilometres (25 miles) high, deep in the stratosphere, fluttered back down to the Alice Springs launch site after it came loose.

Witnesses said they were asked to move out of the way before the payload, containing expensive scientific instruments, was suddenly dragged across the launch site.

"We started moving the cars and just barely made it out without getting smashed," one witness said.

"(There was) debris flying through the air everywhere," said another. "That was it, just an instance of chaos outside."

Scientists last week completed a similar balloon flight to measure X-rays and gamma rays sent out by various stars and galaxies from deep in the Earth's atmosphere.

Ravi Sood, director of the Alice Springs Balloon Launching Centre, said scientists involved in the NASA-sponsored project were extremely disappointed.

"Ballooning, that's the way it happens on occasions but it is very, very disappointing. Gut-wrenching actually," he told ABC.