Doc biodiversity ranger Jim Fyfe said the incident with a Weddell seal occurred at Middle Beach between 2pm and 3pm on Monday.
The seal — a type often found around the Antarctic ice — was injured but not seriously, and the person walking the dog had reported the matter to Doc.
The seal had remained on the beach for about a day and a-half before swimming off, apparently early on Wednesday.
Dunedin City Council dog control bylaws require that "in all areas dogs must be placed on a leash within 20m of protected wildlife".
Mr Fyfe said the onus was on people walking dogs to check the beach, and ensure dogs were kept on a leash to prevent them running off and encountering protected wildlife.
The matter was referred to the city council and a DCC spokeswoman said, at the request of Doc, the council was carrying out an investigation.
