Dean Miller, an Australian fur seal biologist, was the first person to spot the large white object floating past Macquarie island in the far south-west corner of the Pacific Ocean.

"I've never seen anything like it. We looked out to the horizon and just saw this huge floating island of ice," Miller told the Australian Antarctic division. "It was a monumental moment for me as it was the first iceberg I have seen."

Estimated to be about 50m high – from the waterline – and 500m long, the iceberg is now about five miles (8km) off the north-west of Macquarie island, halfway between New Zealand and Antarctica south-west corner of the Pacific Ocean.

Scientists have said it is rare for icebergs to be seen so far north. Neil Young, an Australian Antarctic division glaciologist, said: "The iceberg is likely to be part of one of the big ones that calved from the Ross ice shelf nearly a decade ago.

"Throughout the year several icebergs have been drifting slowly northwards with the ocean current towards Macquarie Island. We know there are also a few more icebergs 100km-200km to the west of the island."

Several huge icebergs broke off from Antarctica's Ross ice shelf and the Ronne ice shelf in 2000 and are now drifting away from Antarctica. The first was about 190 miles long and 23 miles wide.

Scientists believe the iceberg off Macquarie will break up and melt rapidly as it meanders northwards. However, before melting it could present a danger to ships, Young said.

Since the start of satellite records in 1979, total Antarctic sea ice has increased by about 1% a decade, according to Nasa. The US space agency says it is unclear whether the small overall increase in sea ice expanse is a sign of meaningful change because ice areas vary considerably from year to year and from sector to sector.

Considered individually, only the Ross sea sector had a significant positive trend, while sea ice extent has actually decreased in the Bellingshausen and Amundsen seas.