Their antics are proving a huge hit, but a seal pup playground has become so popular authorities are having to consider how to manage the welfare of the animals.

Hundreds of fur seal pups are travelling from the South Island's east coast up the Ohau Stream, about 30km north of Kaikoura, where they frolic under a waterfall, leap and dive through the water and playfight.

Thousands of people are visiting after word has got out about the free access to view the animals up close in their natural habitat.

The Department of Conservation is concerned about an increasing number of people trying to touch the pups, while there have been reports of people taking dogs with them, and tennis balls have been seen in the water.

The pups' inquisitive nature meant they might get up close to study a person "but they are wild animals ... and they can bite, causing very serious injury", said DoC acting area manager Phil Bradfield.

"We would ask people to respect the seals, give them space, don't crowd them and ensure they have an escape route. And especially don't attempt to touch or feed the pups.

"Like a lot of our native animals, who have evolved for millions of years in the absence of people, they show very little fear. And that's why they come up to people."

What looks like a performance for onlookers is actually honing hunting skills for when they go to sea to catch fish, he says.

"I liken it to the fact that they are in their teenage phase of life," Mr Bradfield said.

"They are doing lots of social interacting. They are not so dependent on their mothers, who are going out to sea to feed, which takes two or three days.

"It's an amazing phenomenon to witness."

Information panels were being planned to help people understand what was expected, said Mr Bradfield. He could not envisage restricting access to the seal pups.

But in future years "we may have to manage people a bit closer ... a bit more than we currently do".

The pups, born late last year, travel up the stream over winter and by October few will be left on the land.