Tourists are returning souvenir rocks to a Bay of Plenty island to avoid being cursed with bad luck.

Tuhua, or Mayor, Island near Tauranga is home to a large amount of obsidian, a dark volcanic rock prized by early Maori for cutting tools and weapons.

Local Maori have warned the rock is sacred and removing it could bring bad luck.

Tourists have heeded the warning, sending chunks to Aerius Helicopters, which runs flights to the island, and asking them to return them.

Aerius manager Shane Jeffcoat returned six pieces last week and another two people have since contacted him.

"One lady had a piece for quite a while, it was in her back garden. She was convinced she'd had bad luck because of it," he said.

One package arrived with a note reading: "These stones were found while diving at Mayor Island in the 70s. (Returning them) may bring me good luck."

Jeffcoat advised everyone he took to the island that the rocks should not be taken.

"Unless you are the owner of the island you are stealing it."

Tuhua Island belongs to Te Whanau a Tauwhao ki Tuhua and is administered by the Tuhua Trust Board.

Board chairwoman Magda Williams told the Bay of Plenty Times she had heard bad luck stories from people who had taken obsidian.