Seals are reasonably frequent visitors to the Whanganui River, and one has even been seen as far upstream as Hipango Park.
Young seals with their mothers often come ashore when they are injured or need a rest. The mothers usually return for them.
Conservation Department biodiversity ranger Sara Treadgold is asking people to leave the youngster alone, and also keep their dogs away.
"Don't feed it. Its mother is probably just out and coming back for it. Keep your distance and leave it alone."
Seals can bite, causing serious injury, and they carry some infectious diseases. People encountering them on beaches are asked to keep 20m away, and not get between the seal and the sea.
The department has a hands-off policy with seals and will only intervene if a seal is obviously severely injured, is entangled in marine debris or is in a dangerous place such as on or near a public road. In that case, people could call the 24-hour hotline 0800 DOC HOT (0800 362 468).