The Department of Conservation has condemned the "irresponsible behaviour and thoughtless acts of cruelty".
A man was sentenced last week to four months' jail for attacking a leopard seal in Te Waewae Bay last year.
Last Friday, a sea lion was spotted by tourists with a Hawaiian sling diving spear embedded in its chest on Surat Bay beach, near Owaka.
They reported it to Doc but efforts to find it were unsuccessful.
Doc Owaka ranger Cheryl Pullar said pictures sent by the tourists who found the animal appeared to show the spear was not deeply embedded and it was hoped it would fall out.
Also on Friday, members of the public reported a sea lion with large cuts on the beach between Kaka Point and the Nuggets.
On Saturday, a nesting yellow-eyed penguin was mauled by two dogs at Jacks Bay, also near Owaka, after the dogs' owners were warned by locals penguins were nesting nearby.
The bird's injuries were so serious it died.
"It was sitting on a nest with two chicks, so they are likely to die as well."
Doc also received a report on Saturday a rottweiler dog had harassed yellow-eyed penguins on the beach at Roaring Bay, north of Surat Bay.
Doc Coastal Otago area manager Robin Thomas condemned what he called the irresponsible behaviour and thoughtless acts of cruelty carried out by the people who owned the dogs which killed or injured the protected, endangered wildlife.
"These actions are not only totally illegal and seriously impact the species but, in respect of the sea lion incidents, are also potentially very dangerous."
Doc was investigating the attacks and in relation to the Jacks Bay incident was following leads as to the dogs' owners, he said.
The owners could face serious penalties under the Wildlife Act.
While sea lions were naturally inquisitive, especially when they encountered divers, they usually watched on, then left, he said.
However, aggressive actions of this nature could cause the animal to lash out in response to the pain of being speared or develop a negative relationship with humans.
"It's a big concern if people start acting that way."
He advised anyone who came across a sea lion while diving not to make any aggressive moves towards them, not to make eye contact and move quietly away.
People who encountered a sea lion on the beach should stay calm and avoid engaging with them.
They should also stay well away from nesting penguins and keep dogs on a leash when walking on beaches with resident penguins.
Doc asks the public to report sightings of injured marine mammals and to immediately report any harassment or disturbance of them by phoning 0800 362-468.
