The NZ Transport Agency, Heritage New Zealand and local hapu are excavating what could be a defensive trench from the New Zealand Land Wars or the inter-tribal Musket Wars.
Contractors widening State Highway 29a in Matapihi, Tauranga, for a safety median barrier made the discovery.
The site had previously been identified as being in an area of archaeological significance for Ngai Tukairangi, Ngati He and Ngati Tapu.
Work stopped immediately and the agreed cultural protocols were enacted with the hapu.
The Transport Agency's Bay of Plenty State Highways manager, Niclas Johansson, said the find could be significant.