The witness, who asked to remain anonymous, said they entered the water to pull one of the dogs off the bleeding marine mammal, which remained listless after the March 20 attack.
The witness, a member of a film crew shooting a promotional video at the quarry near Blackhead beach, said the crew were concerned the juvenile sea lion would not survive the frenzied mauling.
Yesterday, the Department of Conservation said the animal had survived the incident.
Filming had stopped for lunch when three off-leash dogs started a commotion around bushes at the beach below, the witness said.
Soon after, the young sea lion emerged from the bush with the three dogs scrabbling around, attacking it.
The owner was unable to control the dogs, all of which appeared to be terrier-cattle dog crosses, and started whipping the animals with a lead.
The sea lion started to slowly make its way towards the water with the three dogs "in pack-hunting mode" clambering all over it and the owner whipping the melee.
Two female backpackers nearby tried to intervene, but appeared to be more traumatised by the incident unfolding in front of them.
At this point a couple of the film crew rushed down to the beach.
"By that time, the seal's at the water being attacked by the f...... three dogs at the water, and it's trying to hold the dogs off.
"But ... the f...... seal's bleeding and the lady's still f...... trying to whip the f...... seal and the dogs and then the two backpackers are kind of screaming and crying.
"Then I just ... grabbed one of the f...... dogs in the water, and pulled the dog off the seal, and then dragged it away."
As the dogs were brought under control, the sea lion remained listless, "just floating in the water".
A police officer who happened to be at the beach started talking to the owner, the witness said.
"It was mental.
"She just had no control over her dogs and she should not have had three f...... dogs on the beach — especially when off leash."
Doc investigation team lead Graeme Scott said the department was aware of the incident.
"The sea lion survived the attack.
"We have spoken to the owner of the dogs involved and have yet to determine an outcome."
