One witness, who did not wish to be named as he felt "threatened" by the alleged offenders, described the incident to the Otago Daily Times yesterday as "disgusting".
The man was visiting Tautuku Beach near Papatowai in the Catlins with his three children and partner last Thursday, when several utes pulled up and began "blasting" along the beach.
He said a large male sea lion emerged from the water and bridled at the driver of one of the vehicles as it drove past.
In response, the driver drove rapidly towards the animal, braking suddenly about 10m away.
The driver then spun his wheels to throw sand over the sea lion, the witness said.
The harassment continued as other members of the group circled the animal on trail bikes in an apparent attempt to make it chase them.
Feeling "threatened and disgusted" by the group’s behaviour, the family left shortly afterwards.
A concerned local said he saw what he believed to be the same group driving aggressively and harassing seagulls at Tautuku the following day.
"Unfortunately, we get it every summer. If you know Tautuku, it’s an amazing, peaceful spot, and it’s a mystery to me why people want to behave like this.
"[Clutha District] Council has a driving on beaches bylaw, but it’s very weak, with tiny signs.
"It would be nice if Doc had a police force that could just walk up to offenders and fine them."
Doc Murihiku operations manager John McCarroll said he was aware of the incident, although the death of the sea lion could not conclusively be linked to the harassment.
"An adult male sea lion was ... reported as found dead on Tautuku Beach - we don’t know if it was the same one.
"It appeared to have injuries, but it is too early to speculate on its cause of death.
"We will be assessing if a necropsy is able to be undertaken, due to difficulties with the location and condition of the body."
Behaviour of the sort described was "unacceptable".
He encouraged anyone concerned by harassment or mistreatment of native species to contact Doc when it was safe to do so, on 0800 362-468.
Police said yesterday they had not been contacted regarding the incident.
