A mud volcano in the Waimata Valley near Gisborne erupted and spewed mud over 1.3hectares.

A mud volcano that erupted on a fenceline between two Gisborne farms is continuing to spew mud a month after it appeared.

A mud volcano is an eruption of mud, cold water and gases. This one, in the Waimata Valley, began on December 15.

Gisborne District Council scientist Murry Cave said the main eruption lasted five hours and spewed thousands of tonnes of mud over some 1.3 hectares, destroying a fenceline and partially burying trees.

MURRY CAVE This boundary fence was a casualty of the mud volcano.

"We've been monitoring the area for the last year or so because the area uplifted in the Te Araroa earthquake [in September 2016] and there were a lot of cracks," he said.

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MURRY CAVE The mud volcano formed between two farms in the Waimata Valley, near Gisborne.

Cave said the cause of eruption was unknown.

It was thought that it may have been related to movement on the boundary between the Australian and Pacific tectonic plates.

"The origins of these things are a wee bit enigmatic, which is why we and GNS Science have a project looking into these things. We'll be setting up a long-term monitoring programme to study it ... we have similar features within the city limits and they are a potential hazard when you have houses nearby," he said.

MURRY CAVE A mud volcano is an eruption of mud, cold water and gases.

This latest eruption was in a remote area, about 150 metres from the nearest house.

Cave said GNS had sensors in the area prior to the eruption and it was hoped that these might shed some light on what led to the eruption.

"There's obviously been something brewing here for a while since the Te Araroa event and it's just gone boom," he said.

MURRY CAVE The mud volcano spewed mud over some 1.3 hectares.

"We're also taking rock samples from it to see what mix of materials is involved so we can get an idea of how far down these things come from."

There was a hard crust covering some of the mud, but most of it was extremely soft.

The eruptions are associated with hydrocarbon gases and these were present in this eruption.

MURRY CAVE A hard crust covers some of the spill, but most of it remains soft.

There was another mud volcano in the area that erupted every 10-15 years, but this was the first in this area in known records.

The eruption is on private land but can be seen from the Waimata Valley Rd.

MARTY SHARPE Gisborne District Council principal scientist Dr Murry Cave, pictured, said the origins of mud volcanoes "are a wee bit enigmatic".

MURRY CAVE The eruption was in a remote area, about 150 metres from the nearest house.