A woman who was attacked by a dog during a pre-Christmas swim at a City of Cape Town coastal resort is in line for a payout from the council.

Fatima Carelse sued in the high court in Cape Town, which ruled on Wednesday that the city council had been negligent.

The owner of the dog, which was confiscated and put down after the attack, will have to share Carelse's damages bill with the city council.

Carelse went to Harmony Park, between Strand and Gordon's Bay, for a day out in December 2013, said Acting Judge Wesley Vos.

After putting up their gazebo, Carelse, her niece Zulfah and friend Emilio Nelson were playing with a ball in a tidal pool when a dog leapt in and "viciously attacked" the woman.

Sixteen-year-old Dylan Eksteen, who had brought the family pet to the pool, dived in to try to control the dog but it also bit him, said Vos.

After first-aid treatment, Carelse was taken to hospital.

Harmony Park has three entrances, and City of Cape Town officials told the court only one of them was staffed by law enforcement officers, whose main role at the time was to prevent visitors bringing in alcohol. A sign there indicated that dogs were not permitted.

Vos said the other entrances, one of which was used by Dylan Eksteen, had no signs.

"The city knew that visitors and dogs entered ... through the two unmanned entrance areas. It took no reasonable steps to prevent it," he said.

Additionally, the city council knew dogs were a problem at Harmony Park, because officials testified that they had to be removed daily.

Vos said the council's failure to put up signs and provide access control at the two unstaffed entrances "was unreasonable and negligent".

The dog's owner, Quinton Eksteen, was ordered to pay half of the damages, which Carelse will now have to prove at a further court hearing.