What is thought to be the largest known sinkhole in New Zealand has ripped open across a farm on NZ's North Island, revealing a gigantic cavernous void estimated to have been decades or even a century in the making.

The sinkhole, situated about 15 kilometres south-east of the city of Rotorua in a region called Earthquake Flat, measures as long as two football fields and plunges to a depth that would swallow a six-storey building.

"The largest I've seen prior to this would be about a third of the size of this, so this is really big," volcanologist Brad Scott from Kiwi geoscience firm GNS Science told AP.

921 biggest sinkhole new zealand 2(RNZ/YouTube)

A farm assistant first came across the sinkhole last week before the sun came up, narrowly avoiding riding into it on his motorbike when rounding up cows for milking early in the morning.

Only later in the day, when visibility was improved, did the sheer scale of the gaping chasm become apparent.

"It wasn't until I came down in daylight that I actually saw just how big it was," said farm manager Colin Tremain.

"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."

According to Scott, the sinkhole could have been forming for up to 100 years, after decades of rainfall slowly eroded the farm's limestone rock foundations.

After a period of intensive rainfall at the end of April that lasted for about a week, the last of the rock's resistance gave way, opening up to reveal this jagged, 200-metre-long (656 ft) ravine that looks like something out of the movie 2012.

The 20-metre (66 ft) depth of the hole isn't just vertigo-inducing. It also offers a fascinating scientific cross-section to geologists, showing the gradual build-up and layering of rock, sediment, and soil over unimaginably long timescales.

"What I see in the bottom of this hole is the original 60,000-year-old volcanic deposit that came out of this crater," Scott told TVNZ.

"Then there's a stack of about 10 to 12 metres of sediment sitting on top of it from lakes that have formed in this crater. The top three metres is volcanic ash."

921 biggest sinkhole new zealand 2(RNZ/YouTube)

What makes this sinkhole so much bigger than ones that have preceded it in New Zealand is the record-breaking rainfall that helped cause it – almost 170mm of rain in 38 hours.

Thanks to that decisive deluge, volcanologists and landslide scientists will be busy over the next few weeks, inspecting the chasm, and seeing what they can learn from it.

Others meanwhile, are just happy this giant void didn't swallow them whole – like the farm assistant, Gabriel Lafalla, who narrowly avoided riding into the sinkhole in the pre-dawn darkness.

"I could have died," he told Stuff.

"I touched myself [the sign of the cross] and said to myself, 'I'm alive'."