Volcanologist labels Rotorua sinkhole the biggest he's ever seen

A volcanologist says a giant sinkhole that's opened up on a Rotorua farm is the biggest he's ever seen.

Sunday's heavy rainfall caused the earth to crumble and cave, leaving a huge surprise for the farm manager.

The huge chasm in the earth opened up overnight in one of Colin Tremain's paddocks.

Its 200 metres long, 20 metres deep, and just as wide in some places. The chasm's size was underestimated by the farm worker who discovered it.

"He told me it was 20 metres long and four metres deep - it wasn't until I came down in daylight that I saw just how big it was," Mr Tremain told Newshub.

The earth is falling away from under the fence line and forming a huge crack up the hillside.

On Wednesday GNS volcanologist Brad Scott came to survey the sinkhole. He's amazed at its size.

"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.

Several fault lines run beneath the land the farm sits on.

This is the ninth sinkhole - or tomo, as they're officially called - to form on the property in recent years.

"The cavity would have been present from all the rainfall events over the last 4000-5000 years, the high-intensity rainfall we saw over the weekend just accelerates the process," Mr Scott said.

Rainfall of146mm fell at the farm on Sunday, triggering the sinkhole.

Witnesses to the tomo can see where the cracks have formed in the road, and it's completely crumbled away. Although the edges will continue to collapse, it isn't expected to grow much bigger.

"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," Mr Tremain said.

But it's an overnight change that has become a permanent feature of this farm's landscape.