A social media-driven craze for so-called "muddy buns", also known as "dirty dirty bread" or "zang zang bao" in China, is driving up demand for butter around the world, according to Fonterra.

The co-op said its Edgecumbe plant was geared up to handle this global trend and is commissioning a new butter line, which will nearly double the factory's butter sheet production from 4500 tonnes to 7000 tonnes.

Fonterra's general manager marketing, global foodservice, Susan Cassidy said "dirty dirty bread" can best be described as a chocolate croissant.

"People love the flaky chocolate pastry that's coated in rich chocolate ganache and sprinkled with cocoa powder," she said.

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"It makes it impossible to keep your face clean while eating," she said in a statement.

"They are popular with celebrities who have taken to social media to share images of their 'muddy bun face' experience."

Cassidy said demand for butter is as strong as ever.

"People want natural products and they are prepared to pay for them. And that's why global butter prices, including here in New Zealand, are continuing to rise," she said.

"Even in temperatures of minus 10 degrees, crowds of people are queuing for hours to get their hands on their Muddy Bun," she said.

Edgecumbe's operations manager, Allan Muggeridge, says the first butter sheet will roll off the new butter line on September 1.

"We've been watching demand for butter build for a number of years now. The building part of the project started in May so it's been a quick turnaround to get it up and running," Muggeridge said.

Fifteen local contractors have been employed to complete the expansion at the site, which employees 380 people.

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"It's been a real boost to the local business community, especially after the rough past 12 months Edgecumbe has had," Muggeridge said.

Prices for butter have been very strong in recent months, going on latest Global Dairy Trade auction results.

At last week's sale, butter prices hit US$5390 a tonne, high by historical standards but down from their GDT record peak of US$6026 a tonne in September last year.

The lowest GDT price butter over the past five years was $2293 a tonne in August, 2015.