The herd becomes agitated and begins to approach the man as the bull roars.
Carrying his surfboard, the long-haired man charges directly at the mammal, coming within a couple of metres of the bull before turning away.
He then picks up a small object, which appears to be either a stone or a shell, and throws it towards the clearly distressed animal.
Speaking on condition of anonymity, the New Zealand Sea Lion Trust volunteer who captured the footage said she hoped the surfer would be prosecuted for goading the sea lion.
This week, several incidents of people harassing protected New Zealand sea lions have come to light, including one at nearby Sandfly Bay, where a young woman dances within metres of a bull, and another at the same beach where a man poses close to a sea lion for a photo.
New Zealand Sea Lion Trust chairwoman Jordana Whyte, of Dunedin, said people needed to realise they were lucky having the marine mammals on their doorstep and treat them with respect.
She encouraged locals to model good behaviour around sea lions for tourists.
''In Dunedin we're really privileged because we see them all the time.
''But they're actually a nationally critical animal ... we're very lucky to have them here.''
Doc biodiversity ranger Jim Fyfe, of Dunedin, said the surfer's behaviour had left him open to prosecution under the Marine Mammals Protection Act.
''It's always disappointing to see people needlessly harassing these animals.''
The signage at Sandfly Bay was due to be upgraded soon, he said.
NZ sea lion facts
• Global population: about 12,000.
• Conservation status: nationally critical.
• Habitat: most of the population are found at the Auckland and Campbell subantarctic islands, while Stewart Island, Otago and Southland are emerging breeding areas.
• Advice: stay 20m away from active sea lions, and 10m away from resting animals.
