Mr Brown said anyone encountering a leopard seal - or any seal - should keep at least 20m away and not disturb them.
"Leopard seals can be aggressive if agitated and while they can appear docile resting on shore, they can lunge powerfully and quickly," he said.
"If a leopard seal is basking on the beach or swimming around in the sea and is fine then people should just leave it alone.
"People should never get between a seal and its access to water."
Reader Robin Tulloch contacted the Herald after coming across the seal while out for a walk with her dog.
"I noticed what looked like a banana-shaped log. The dog didn't notice it at first as she was sniffing around on the grass on top of the sea wall.
"The dog then jumped and looked at this log from directly above and I became aware that perhaps it wasn't a log," she said.
"I tied the dog up and stood directly above it. It was a large seal. It had a white underbelly and a sleek face like a snake. No ears.
"It had spots around its neck and under jaw. It was at least two metres long. It appeared to be healthy. There were no visible injuries on its grey/green body.
"It looked at me with large eyes [and] opened its mouth to show a row of teeth. It then turned over in the sand and using its flippers entered the water and swam away out into the bay. It surfaced well out into the channel. I saw it surface once and then it disappeared.
"A startling start to my day and a wondrous experience."
Mr Brown said leopard seals had long, slim bodies, comparatively large fore-flippers.
"The disproportionately large head, massive jaws, impressive teeth and tremendous gape give them a snake-like appearance.
"It's rare for a leopard seal to make it as far north as Auckland as they are normally found along the edge of the Antarctic pack ice."
Mr Brown said the seal spotted yesterday was probably a lone juvenile male.
Antarctic visitor a solitary predator
* Leopard seals are usually solitary animals.
* They are known to prey on a variety of species including penguins and other seals.
* They are the only seals known to regularly hunt and kill warm-blooded prey.
* Large adults have attacked humans.
* The leopard seal is built for speed.
* Females can grow up to 3.6m and weigh up to 450kg.
Males can grow up to 3m and weigh 270kg.