An investigation is underway into the death of four Hector's dolphins on South Island beaches

Three were found in the past week.

A dolphin calf has been found at Marfells Beach in South Marlborough and another baby near Hector on the West Coast.

One adult was found north of Hokitika and another at Okarito

DoC aquatic director Elizabeth Heeg says they don't know why they died but they're grateful to the public for reporting them.

Hector's are the world's smallest dolphin species and are considered vulnerable.