A massive sinkhole has opened up on a New Zealand farm, with a volcanologist saying it is the biggest he has ever seen in the country.

The crack was discovered at Earthquake Flat, 15 kilometres south-east of Rotorua on New Zealand's North Island.

Vulcanologist Brad Scott visited the farm where it was discovered and said it would have taken decades for the underground cavity to grow, but was spurred on by recent heavy rain.

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

"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."

Farm manager Colin Tremain said he could not believe the size of it.

He said the area often developed sinkholes due to degrading limestone rock beneath the ground's surface.

"It wasn't until I came down in daylight that I actually saw just how big it was," he 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."

ABC/AP