"I didn't know whether it had just scratched me or bitten me so I carried on swimming and I looked around and he was following me again.
"I shooed him away and he went back to shore so I just carried on to the other buoys and then got out. When I got out it [the wound] was bleeding heavily."
Scanning the beach, she found the dog's owner and approached the woman to show her the wound.
"I was bleeding and I said it wasn't very good. She apologised and said he'd never done it before.
"I think she was a little bit shocked as well but I thought she could have offered to accompany me to the emergency department or at least walk me to my car or something."
Napier City Council regulatory solutions manager Hayleigh Brereton said the beach was an on-leash area and signs advised leads were required on dogs at all times.
She said the council was investigating but needed to identify the dog owner before making any decisions about enforcement actions.
"We've tried to identify the dog or the owner from what we know so far but would be very grateful to hear from any members of the public that witnessed the event, or could identify the owner. Equally we'd love to hear from the owner."
Shrider said she had since paid a medical bill of $47 and had to get four stitches in her leg.
Having only recently started swimming at Sandy Beach, drawn to its calm waters, she said she would now avoid the spot when lots of dogs were there.
"It wasn't nice at all. If it was a shark you'd expect it, if you were in shark-infested waters, but I didn't expect to be in dog-infested waters.
"I was utterly surprised and totally shocked. It's just not what you expect to happen when you're going out for a swim. It will just make me very wary now.
"If there's any dogs on the beach, I won't be going in the water."
Ms Brereton said the council encouraged dog owners to take their responsibilities under the Dog Control Act seriously and ensure their pets were under control at all times.
Yesterday, a man was attacked by a dog described as a rottweiler at about 11am at Ocean Beach, a Hastings District Council spokeswoman said.
The man was playing with his two dogs, when the rottweiler started attacking one of the animals.
"He tried to stop it and he got serious injuries to his hand."
She said, "witnesses believed they knew where the dog lived or was staying but by the time our dog team checked no animals or people were at the address''.
Inquires are ongoing.