Thousands of round and oval balls, ranging in size from a tennis ball to a basketball, have littered the beach for the past couple of weeks, along with a few sausage-shaped cylinders.
Plenty have been taken home by beachcombers, but there is no shortage of replacements.
Invercargill science teacher and naturalist Lloyd Esler said the balls were marram grass roots ripped from the sand dunes by heavy seas, then tangled and ''felted'' as the surf rolled them about.
Marram grass roots were bristly and spiky and could be up to 8m long, he said.
The balls and cylinders are bristly too, leading Mr Esler to describe them as ''hedgehogs''.
''They quite interesting, aren't they. They're all natural, so they would make great firelighters or compost for the garden.''
He said he preferred to play with them.
''When you see something the size and shape of a football the human instinct is to kick it.''
Mr Esler said the phenomenon occurred once or twice a year on Oreti Beach.
He had also seen similar balls at Mason Bay, on Stewart Island, and on beaches on the west coast of the North Island.
