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Evgeny Rodygin was on Smaills Beach last week when he saw the dog approach and begin barking at a resting sea lion.
He took a photo and posted it online to remind people to put their dogs on a leash.
"I think that everyone should respect that the beach is wildlife territory and follow common-sense rules," Mr Rodygin said.
Department of Conservation (Doc) marine and coastal species liaison officer Chris Page said it was upsetting to see, especially as it was preventable.
Dog interactions were stressful for sea lions and Doc had seen dog attacks in the past that caused "gruesome" injuries and even killed marine mammals.
"We are lucky to have so many marine mammals, penguins and shorebirds frequenting our coastline — we need to be better at sharing the space," he said.
It was in everyone’s best interest to ensure beaches were shared and dogs were kept under control, as they could also get injured or contract illness from wildlife.
Mr Page said dog owners should always carry a lead and scan the beach thoroughly before releasing their pet.
People should also stay at least 10m away from sleeping sea lions and 20m away while they were awake.
New Zealand Sea Lion Trust education adviser Hanna Ravn said harassment of wildlife was an illegal act, so it could be reported and prosecuted.
"We try the best that we can to educate in a positive way, but sometimes we do wish that there could be more back-ups from the legal side."
It was important to always be considerate towards sea lions.
"The female sea lions are travelling sometimes hundreds of kilometres a day to go and fish and then come back to the shore."
If the sea lions did not feel safe to come ashore to where their pups were that could have some "big consequences", she said.
At times, the mothers were also gone for days at a time so when they finally returned it was important to feed the pups quickly.
Ms Ravn said people could also help keep sea lions safe by letting the trust know when they saw a sea lion by reporting its tag number, but only if they had a camera with a good zoom or binoculars to read the tag without getting close.