Australian authorities on Friday issued a shipping alert over a gigantic iceberg almost twice the size of Hong Kong island that is gradually approaching the country's southwest coast.

The Bureau of Meteorology said the once-in-a-century cliff of ice, which dislodged from Antarctica about a decade ago before drifting north, was being monitored using satellites.

"Mariners are advised that at 1200 GMT on December 9, an iceberg approximately 1,700 kilometres south-southwest of the West Australian coast was observed," it said, giving the iceberg's coordinates.

"The iceberg is 140 square kilometres in area – 19 kilometres long by eight kilometres wide."

Experts believe the iceberg -- known as B17B -- is likely to break up as it enters warmer waters nearer Australia, creating hundreds of smaller icebergs in a hazard to passing ships.

"It's still 1,700 kilometres away, so it's quite a long way away, it's not really on our doorstep yet but it's been heading steadily towards us," glaciologist Neal Young said yesterday.

Young earlier said that an iceberg of that size had probably not been seen in the area since the days when 19th century clipper sailing ships plied the trade route between Britain and Australia.