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UPDATED 8.43PM A large fireball which flashed across the South Island this evening was not a meteor, but debris from an orbiting spacecraft, a leading astronomer says.

The red fireball was spotted by hundreds of people when it entered the earth's atmosphere about 6.30pm.

Former resident superintendent of Canterbury University's Mt John Observatory, astronomer Alan Gilmore, said the fireball was rarer than a meteor.

"It's not a meteor, I'm certain of that. It took too long to go across the sky."

Mr Gilmore said the flash had all the characteristics of a re-entry of debris from a space craft, or piece of equipment which had been orbiting earth.

Mr Gilmore said the only other such case in Southern skies he knew of happened about 10 years ago.

WeatherWatch head analyst Philip Duncan said after viewing video footage, he too no longer thought it was a meteor.

"It seemed quite slow-moving. Perhaps it could be a rocket or satellite re-entry,'' he said.

"Or it may just simply be the long low distance making it look slower and may well be a meteor. I'm a bit more unsure now.''

Waimate man Ralph Pfister was driving home from work when he spotted a peculiar object in the sky.

"It looked like a plane coming, but there were all these embers - like sparks - coming from the back of it.

"It kind of popped - like an explosion, but there was no noise - and then it exploded into smaller pieces.''

WATCH ABOVE: Callum Law shot this raw footage