Spiders have begun falling from the sky in south-western Victoria, as cooler conditions make way for a phenomenon known as ballooning.

The spiders sought out higher ground after wet weather, spinning a sea of cobwebs that covered the ground on a dairy farm in Gorae West, outside Portland, earlier this week.

In a similar occurrence, millions of spiders invaded farmland in the New South Wales Southern Tablelands last month.

McLean View Dairy director Nathan McLean said spider silk covered the property.

"You ride across the paddock through the webs on a four-wheeler and even though it looks like it's on the ground you'll get spider webs around head height," he said.

"We tend to see it a lot in late autumn, early winter."

Australian Museum naturalist Martyn Robinson said it was common for the spiders to seek higher ground during the onset of cooler temperatures and heavy rain.

"The baby spiders will climb to the top of the nearest tall object, they stick their abdomens in the air and they release silk," he said.

"When the tension in the air is strong enough the spider simply lets go, and they get carried along in the breeze until they either drop the line and fall to the ground or the breeze drops.

"They can travel along in that way for kilometres – they have devices like a hoover powered by a battery – and they haul themselves up like a hot air balloon.

"People have caught baby spiders up to three kilometres off the ground."

Mr Robinson said early winter provided the optimal conditions for viewing the phenomenon.

"You'll get a lot of spiders hatching out in the early May and June period, and then you'll get the fogs and mists and things forming, so that combination means it will be fairly regular," he said.

"It's a fairly common technique for baby spiders ... and also ground-dwelling spiders.

"It's no coincidence that spiders are some of the first colonisers of new islands, because they can just drift along so effortlessly and snag lines on new volcanic islands that have risen up in the past couple of years."