World champion wood chopper Jason Wynyard needed more grunt so he ripped a cylinder from a snowmobile to create what is probably New Zealand's most powerful - and loudest - chainsaw.

Wynyard was yesterday showing off his skills and enormous chainsaw at the 43rd National Fieldays being held at Mystery Creek.

The largest agribusiness exhibition in the Southern Hemisphere with more than 1000 exhibitors, the Fieldays runs until Saturday.

Among the hundreds of attractions was Wynyard's 32kg hot saw which - with a 20-inch bar, a 60hp motor and a 300cc engine capacity - had the crowds scampering in and just as quickly covering their ears.

Wynyard came up with the idea for his beast-like chainsaw while competing at the Stihl Timber Sports series in the United States in the 1990s.

The hot saw was one of several new disciplines he had to master for the series so he took a cylinder from a 1999 Rotax 600cc twin snowmobile, cut the crankshaft and had the rest of his chainsaw custom-made.

"I finally got it built and she's a bloody ripper ... it wasn't cheap though, it probably cost about $18,000."

Wynyard said the chainsaw was for show and wouldn't cut the mustard in terms of practical use.

"You probably wouldn't want to use it for firewood," he said.

"It's purely for show and designed for that, you can't really run it for more than five or six seconds."

Meanwhile, Skellerup marketing executive Deborah Allan said despite the unusually warm weather and lack of mud and puddles because of the newly-laid tarseal around the grounds, the farmer's essential accessory, the gumboot, was still flying out the door.

Ms Allan said her staff had brought a 40-tonne container full of gumboots to Fieldays and expected to sell the lot.

The Red Band, which has not changed in design in the past 50 years, remains the most popular for farmers but is being challenged by a newcomer - the insulated and waterproof Quattro.

"Some of the farmers tell me wearing a pair of those is like wearing slippers," said Ms Allan.

Prime minister John Key said it was an "outstanding time to be a farmer" and despite the high exchange rate New Zealand commodity prices were high and in demand globally.

"The picture here is one that is not just good for 2011," he said.

"We are seeing a big increase in worldwide demand and that bodes very well for New Zealand as a country, whenever we have had very high commodity prices New Zealand has prospered."

Fieldays president Lloyd Downing said he would be happy with crowds to match the 122,000 that attended the event last year and he expected farmers to spend more.

"All the stars are in line, we have had a good growing season and good prices for our products. The banks will take a bit but at the end of the day there will still be enough left for the replacement of machinery."

Mr Downing said 18 foreign embassies and more than 250 overseas business people were to visit this year's event, including delegations from China, Malaysia, Australia, Japan, Ireland, Chile and Germany.