The Awakino Tunnel connecting Waikato and Taranaki, through which traffic has passed for almost 100 years, is about to close.
Early next week, traffic will be switched to a new section of State Highway 3 that bypasses the single-lane tunnel and takes cars over two bridges across the Awakino River.
The Transport Agency says that over the next week or two, traffic may alternate between the tunnel and the bridges as construction work continues.
All traffic is expected to be permanently switched to the new road next month.