He said anyone who encountered a potential explosive device should keep clear and call the police.
Niki Conrad, lead taio ranger for Te Aupōuri, said the device appeared to have washed up on the northern end of Great Exhibition Bay, in the vicinity of Te Kao, in the recent storm.
The iwi contacted local residents urging them to stay away until the bomb squad had made the area safe.
The rangers had been on their way to check a dead sperm whale further north when they spotted the device on the beach.
Marine markers typically contain phosphorus and an explosive charge.
They can be dropped from aircraft or launched from ships, and are used in search and rescue missions, anti-submarine warfare and maritime research.