New Zealand sinkhole: Huge chasm opens up overnight, creating abyss thought to be country's biggest ever

'The largest I've seen prior to this would be about a third of the size, so this is really big'

A sinkhole thought to be New Zealand’s biggest ever has appeared on a farm in the country, revealing a chasm as deep as a six-storey building.

The hole, nearly as long as two football fields, has been forming unnoticed for up to a century, experts believe.

They say several days of heavy rainfall were the trigger for it to open up.

The farm, near the town of Rotorua on North Island, in a region called Earthquake Flat, has had sinkholes nearby before but not of this size.

GNS Science volcanologist Brad Scott said: "The largest I've seen prior to this would be about a third of the size of this, so this is really big.

"The cavity would have been present from all the rainfall events over the last 40, 50, 100 years.

"The high-intensity rainfall that we experienced on the weekend just accelerates the process."

Mr Scott told TV New Zealand that he could see at the bottom of the hole "the original 60,000-year-old volcanic deposit that came out of this crater".

A farm worker found the chasm when he was rounding up cows for milking before dawn, but said he only realised quite how big the hole was when he saw it in daylight.

Colin Tremain, the farm manager, said the area often develops sinkholes as limestone rock beneath the ground's surface degrades.

Mr Tremain said: "We'll keep it fenced off as it is to keep stock out, although stock aren't stupid - they're not going to walk into a hole. They can spot danger."

Sinkholes are caused when layers immediately below the Earth’s surface shift over months or years. They can range from a depression in the ground up to holes hundreds of yards deep.