Betty Ratcliffe has spent four years beachcombing Narrabeen’s shoreline at dawn, but she’d never spotted the bright yellow belly of a washed-up weedy seadragon. Then, in a week, she found seven.

“The first one I found had recently died; it was so vibrant, with orange, yellow and purple,” she said. “Over the next couple of days I kept finding more and more.”

Seadragon expert Dr David Booth, a professor of marine ecology at the University of Technology Sydney, said more than 20 seadragons had washed up on Sydney beaches over the past fortnight.

“I’ve only ever seen one washed up,” he said. “It was like, ‘My god, what’s happening?’”

Dead weedy seadragons were found on the northern beaches, in Cronulla, Malabar, Botany Bay and on the Central Coast. Booth said it was not uncommon to find one or two among the flotsam after a storm, but he had never seen anything like this. He estimated that if 20 weedy seadragons had been found and reported, it was likely at least 50 had washed up in the Sydney area alone.

“I reckon it’s about 10 times the normal rate of wash-ups,” Booth said. “Clearly it’s a result of some combination of the shocking weather, pollutants being washed into the ocean and big surf.”

Weedy seadragons are “tough little devils” that can hang on to kelp in strong currents and live at a depth of about 10 metres, feeding on mysid shrimp, which are “basically tiny prawns”.

Usually, only the top three metres of seawater are affected by a storm, but Booth said it was likely particularly wild surf churned pollutants and muck down through the water column to the seadragons’ haunts.