Two seemingly sickly seal pups on South Island beaches have caused a stir on social media this week.

A baby fur seal on a Nelson beach and another in Invercargill were left for long periods of time despite looking lethargic and, some worried, so sick they could be dying.

DOC received a call on its hotline shortly before 3pm on Saturday about a seal pup on Tahunanui Beach, Nelson, a department spokeswoman confirmed.

The caller was concerned the seal was being disturbed by people around it and a DOC ranger responded and moved the pup from the beach because of the attention it was getting from people.

The seal was taken to a Waimea Estuary coastal spot near Rabbit Island.

In Invercargill, a member of the public had been told yesterday they could move another pup away from the tide line if they were worried about it being washed out to sea, and had been instructed how to do so by a DOC ranger.

DOC was today heading out to the beach to check on the pup.

One man shot a Facebook live video venting his frustration after he felt the Department of Conservation (DOC) didn't act fast enough after he called concerned on Saturday.

But the government department says it's not rangers' job to look out for the welfare of individual members of thriving species.

Animal welfare group Helping You Help Animals (Huha), said all animals had the right to life, and death, with dignity and DOC should have responded more quickly to calls from the public, euthanising the animals if necessary.

"When an animal is found suffering and in pain and there's a department in a position to mitigate any suffering we can't understand why they wouldn't even look into concerns from the public," said Huha founder Carolyn Press-McKenzie.

"If the public is concerned enough to call, it's at least DOC's duty of care to look into it and see if assistance or care is needed."

DOC staffer Laura Boren, who has studied fur seals at PhD level, said while it could be upsetting to see a wild animal which looked unwell, it was not always appropriate for DOC to step in.

The department took a hands-off approach to fur seals, whose population was currently thriving, Boren said.

Often animals were not sick, merely a bit skinny and tired, and would return to the water on their own.

"With that increase we're going to see natural causes of mortality which is going to include seals that aren't able to forage for themselves, or their mothers have died or can't feed them effectively," she said.

"We don't want to make an assumption it's not going to survive, because we could get it wrong."

Where animal populations were self-sustaining, like fur seals are, DOC staff did not have the same responsibilities to individual animal welfare as someone like a vet, she said.

"DOC's responsibility is more for where populations are in danger for human-related issues.

"We'll step in when it's something like an entanglement or the seal's got a hook in it's mouth.

"When a population is doing well that's when we need to step back and let them sort themselves out."

Boren acknowledged members of the public could find seeing a seal they thought was sick distressing, but said baby animals dying was part of nature which we were sometimes sheltered from.

However she said if the public did make a call to DOC, photos and video were helpful so staff could assess the animal's well-being and "triage" the case before deciding whether to head out or not.