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A volcanologist says a giant sinkhole that opened up on a Bay of Plenty farm is the biggest he's ever seen in New Zealand.

The hole, also known as a tomo, was 200 metres long and 20 metres deep, and was now threatening Fonterra's road access to the Tumunui dairy farm.

GNS Science volcanologist Brad Scott said the sinkhole - the largest he had ever seen in New Zealand - had the potential to get even bigger.

"This will erode back, the sides will continue collapsing and the hole will open over the next decade or so."

It was also a bit of a science experiment, revealing soil that was 60,000 years old, he said.

Layers of soil that could be seen included sediment from the most recent Lake Taupo eruption, about 1800 years ago.

Sinkholes are caused by water dissolving limestone or other surfaces. In this case, the volcanic terrain consists of pumice.