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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.