Spectacular spider webs near Hikurangi.
Spiders emerging from flooded farmland have festooned about 75 metres of both sides of Jordan Valley Rd at Hikurangi with their webs.
Untold millions of the arachnids are swarming over the webs and road, with others on bits of web floating away in the wind.
Watch: Video of silk spider web
Hikurangi Swamp farmer Ben Smith said it happened every time the swamp flooded in late autumn. "With 5600ha under water, there's nowhere else for the spiders to go."
Mr Smith said the unusual natural phenomenon usually lasted two or three weeks, with the spiders dispersing as floodwaters receded.
Northern Regional Council biosecurity officer Ross Johnson said baby money spiders were behind the large amounts of a silk-like substance floating in the air and settling on the land.
Photos: Spiders spin wicked web
Image 1 of 14: Millions of spiders on the move across the Hikurangi Swamp on Jordan Valley road create a series of spectacular webs over road markers, road verges, trees and fences. PHOTO/JOHN STONE NAG 13Jun1
Mr Johnson said it happened about late autumn and early winter when spiderlings - baby spiders - threw out a long single thread of silk.
"It's an attempt by the spiders to be picked up by thermals and light winds, allowing them to be transported long distances in a clever natural process called ballooning."
The threads of silk do not last very long and the spiders attached to them posed no threat, he said.
Mr Johnson said he had seen it before and it was a wonderful sight, with paddocks, trees, tractors and anything in their path becoming draped in a blanket of fine silk.
Spiders could use the method of ballooning to cross the ditch from Australia.
Spiders could actually make several different types of silk and the ballooning process involved a different type of silk than that more commonly seen in webs, he said.
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