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.
"We expected to find kumara and fire pits and we have an authority from Heritage New Zealand for the works, but we did not expect to see something like this.
"This could be nationally significant so it's important that we keep the site closed and take the time now to find out as much as we can so we can honour the tangata whenua who were affected by this part of our history."
Project archaeologist Sian Keith said the site would be excavated and all details recorded so a fuller picture of what has been found could be painted.
"The trench is about 40m long and appears to have been used as a coastal defence around the 1830s to 1860s.
"It is away from other well-known battle sites around Tauranga and depending on the date it could add important information to the national historic record," Keith says.
The hapu and the Transport Agency would continue to work in partnership to ensure the correct cultural practice is carried out.
A representative from Ngai Tukairangi, Ngati He and Ngati Tapu will remain on site to monitor the excavation.
The speed limit will remain at 70km/h through the construction site to decrease the risk of serious crashes.
The median barrier project is expected to be completed by the middle of this year.
Discussions will also continue between the Transport Agency, the hapū and Heritage New Zealand on how the site might be permanently acknowledged.