The mass deaths of toxic marine life off Western Australia's Pilbara has prompted warnings from authorities.

In recent weeks, residents have discovered hundreds of dead puffer fish along Onslow Beach, more than 1,300 kilometres north of Perth.

The stretch of coast is popular with dog walkers, swimmers, and fishers from the nearby industrial town of Onslow.

Puffer fish are highly toxic to animals, and can cause severe illness or death if even a small quantity is ingested.

The Shire of Ashburton issued a public health notice on Wednesday, urging people to keep their pets leashed and report any further sightings for clean-up.

Onslow resident Sandi McAullay described the sight as a worry.

"The puffer fish, I've never seen before, and I've been on this beach … 60-odd years," she said.

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