© Steve QuirkA meteor the size of a cricket ball travelling at a typical speed of 20 kilometres a second can light up the ground as brightly as the moon
Early risers who spotted a greenish-yellow light flashing across the shire on Monday probably witnessed a meteor.

Surfers at Cronulla Beach who saw it moving from east to west over Kurnell at 5.20am said it lasted about five seconds before it split into three and disintegrated into a vapour trail.

''I knew it wasn't a shooting star; it was heaps bigger and much closer,'' said one of three surfers who saw it. ''It looked like a ball-shooter [fireworks].''

A Sydney Observatory blog reported 13 sightings from Wollongong to Woolgoolga, 600 kilometres north.

The Australian National University astronomer Rob McNaught, a specialist who searches for asteroids that could collide with Earth, said the sighting was most likely a meteor or fireball ''belly-flopping'' into the atmosphere at 30 to 40 kilometres a second.

It was unlikely to be space junk, such as debris from spent satellites, which orbits the Earth from west to east and is less bright.

''[The meteor] is slowed down by friction with the air so rarely hits the ground,'' he said. ''Meteors hit the Earth's atmosphere faster in the morning than in the evening. Most meteors are just the size of peas, or smaller. Larger meteors are typically fragments from collisions between asteroids.''

Meteors are rarely seen closer than 20 kilometres, or higher than 80 kilometres, from Earth as by then they would have slowed to free-fall speed and would not be glowing.