It’s the stuff of nightmares.

At the same time, rising floodwaters surrounded Melanie Williams’ home, thousands of spiders scaled the fence in her front yard.

“That was enough to really freak me out, I had never seen anything like it before,” she said.

“I am an arachnophobe from way back so I hope they’ve gone back to wherever they came from.

“I occasionally see spiders around the place but never anything like that, it was just insane.”

Ms Williams and her family became trapped in NSW’s flooding crisis at the weekend as a torrent surged down the Nambucca River.

By 1am Saturday, her home at Macksville, on the NSW mid-north coast, was surrounded but luckily the water level began subsiding about 50 centimetres from the back door.

“As the water was rising, the letterbox was going under further and further and I could see all these little black things on there and I thought ‘oh my God, they’re spiders’,” she said.

“Then I looked at my neighbour’s fence and almost had a heart attack. There were literally thousands of them.”

Melanie Williams, with her husband Geoff and daughter Hannah. Photo: Supplied

A plague of spiders might seem apocalyptic, but experts say the episode is easily explained.

Professor Dieter Hochuli leads Sydney University’s integrative ecology group and has made a career out of examining what drives the ecology of animals and plants.

He described the phenomenon as “fascinating” and said the spiders were always there – we just don’t usually notice them.

“All this is happening under our noses, but we just don’t know what’s going on,” he said.

“There’s this vibrant ecosystem happening all the time.

“What happens with the floods is all these animals that spend their lives cryptically on the ground can’t live there anymore.

The spiders on the move. Photo: Melanie Williams

“The spiders are the really obvious ones as they throw out their webs.

“Just like people, they’re trying to get to higher ground during a flood.”

The spiders, which are various species, usually live in long grass or plants, away from prying eyes.

“One of the reasons we don’t notice these sorts of species normally is because they’re hunted by other things, like birds,” he said.

“The urgency of getting out of the floods means these ground-dwelling animals have to move.

“Some spiders can survive underwater for a period of time but others can’t and those ones are basically trying to move from the ground floor to the penthouse to avoid being flooded out.

“They can’t fly, so they do what they have to, to escape the water.”

The water from Tilly Willy Creek made it almost as far as the Williams’ front door. Photo: Melanie Williams

-ABC