The first leopard seal of the season arrived in Dunedin yesterday, and it had already been put in danger before the day ended.

Department of Conservation biodiversity manager Jim Fyfe said about a dozen dog prints, some as close to 5m, surrounded the seal as it lay on Tomahawk Beach.

The juvenile seal, which he believed was a female, was about 2m long.

Fyfe was "disappointed" the endangered seal had already been harassed.

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"It's disappointing and frustrating that it's still happening and dog owners don't appear to know what 20m away really looks like."

On Wednesday Bart Acres, of Dunedin, witnessed a dog attack a sea lion on Smaills Beach, in Dunedin.

"It was probably over a minute the dog was literally jumping on the sea lion and barking at it and biting its neck and the owner was just vaguely calling it."

A post by Acres on the Dunedin News Facebook page asking people to be more mindful of wildlife on Dunedin beaches was liked more than 400 times and attracted 93 comments in support.

Acres did not get the licence plate number of the dog's owner, but hoped people would make more of an effort to watch their dogs.

Fyfe said people should take "20 big steps" away from wildlife on beaches.

"People just need to be really really vigilant going into these places and they have to be aware there is a high chance wildlife could be there."

Dunedin City Council animal control officers would visit Tomahawk beach to ensure the leopard seal was safe.

Anyone caught breaking the 20m rule could face a $300 fine, Fyfe said.