Mr Duffy said people should avoid them because they were dangerous and had been known to act aggressively towards divers in Antarctica.
"Although the animal at Kohi is a juvenile, people should still keep their distance from it and avoid swimming or kayaking near it."
Leopard seals are the most predatory seal, with a diet that consists mainly of penguins and other seals.
Nick Hirst of DoC said the fact seals were recolonising New Zealand's beaches was an encouraging sign for the health of the country's coastline.
"New Zealand seals are a natural part of our coastal environment." But people needed to remember seals were wild and it was best to watch from a safe distance of about 20m.
The two furry invaders yesterday appeared to have entirely different aims. The leopard seal was spotted having a lazy day in the sand at Kohimarama, while the other headed all the way to Middlemore Hospital.
Amanda Provan, 22, who works in Kohimarama, said that when she first saw the seal at 9.30am, there was a crowd of about eight people. "It is still there now, and the crowd has just been getting bigger and bigger.
"It's the first time I have ever seen a seal on Kohimarama Beach, which is very exciting."
It looked happy and healthy, she said. "It was getting some sun rays and just having a good sunbathe."
The fur seal was waiting outside Middlemore Hospital as the early morning shift was about to start.
Counties Manukau District Health Board spokeswoman Lauren Young thought the seal might have got there via the estuary at the back of King's College.
"He was getting a bit grumpy, perhaps because he might have been waiting there about six hours for admission," she said.
The Department of Conservation arrived about 8am yesterday and returned the seal to the sea, she said.
Seals in New Zealand
• The last leopard seal to visit the Auckland area was in Glendowie in 2013, and before that in Herne Bay in 2011.
• Between 100,000 and 200,000 fur seals are estimated to live in New Zealand, compared with about 2 million in the early 1800s. A law to protect seals was passed in 1875 after numbers dwindled because of sealing.
• Non-breeding colonies of New Zealand fur seals are found around Oaia Island and the Poor Knights Islands.