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Niwa scientists believe they have traced the history of a flotilla of icebergs discovered drifting about 260km south of Stewart Island. Oceanographer Mike Williams said the 100-odd icebergs were likely descendants of A-43, a superberg that broke off the Ronne Ice Shelf, south of the Falkland Islands, in May 2000.
At 167km long, A-43 would have stretched from Auckland to Whangarei. Its 32km width would have spanned the distance between central Auckland and Takanini.
But A-43 didn't last long, splitting apart as it drifted into the Scotia Sea, in the southern Atlantic.
By January 2005, it measured about 51km by 21km and had a new name, A-43A.
Niwa believes A-43A then drifted east, about 13,500km on a circum-polar current around Antarctica, eventually washing up between the Auckland Islands and Stewart Island.
"Based on A-43A's last recorded position, and assuming it travelled the shortest possible route around Antarctica, we calculate an average speed of 0.9km/h or about 21.5 kilometres a day," Dr Williams said.
"Somewhere along the way, A-43A has broken into smaller pieces."
The icy armada is now believed to be heading northward, caught by the Southland Current, which runs up the east of the South Island to about Mid-Canterbury before flowing out towards the Chatham Islands.
The largest iceberg is estimated to be about 2km long by 1.5km wide, and by Dr Williams' reckoning contains enough fresh water to fill 1 million Olympic-size pools.
The iceberg rises about 100m above the ocean, but Dr Williams said about 90 per cent of an iceberg's mass was below the waterline.
And one canny Southlander has already arranged flights to view the icebergs.
Despite its size, the monster berg is probably riddled with holes and "catastrophic failure" could see it "just fall to pieces" relatively quickly, Dr Williams said. The flotilla would probably drift about 300km up the east coast of the South Island before veering back out to sea.
The icebergs could be visible from the Otago coast in the next 10 days, but the largest would likely break up before then as it would bottom out on the seabed as it entered coastal shallows.
* An earlier version of this story and graphic wrongly said the icebergs went west. They were forced east by westerly winds.
