Australian seal experts noticed the 700 metre long, 350 metre deep slab five miles off the coast of Macquarie Island, which lies halfway between Antarctica and New Zealand.
“I’ve never seen anything like it – we looked out to the horizon and just saw this huge floating island of ice,” fur seal biologist Dean Miller said.
It is likely to eventually melt into dozens of smaller parts which could be dangerous for ships in the area.
Glaciologists believe the iceberg could be one of the slabs which split from a major Antarctic ice shelf in 2000 and blame climate change.
Other chunks have drifted away from Antarctica but is unusual for them to travel so far towards warmer waters.
“It’s rare to make a sighting like this – it’s certainly impressive-looking,” Neal Young from the Australian Antarctic Division explained.
“If the current trends in global warming were to continue I would anticipate seeing more icebergs and the large ice shelves breaking up.”