Just one lane of the main road between Taranaki and Waikato will be open for at least the next two months.
A rockfall last weekend closed State Highway 3 south of Mokau for four days.
Since yesterday one lane has been open, but the New Zealand Transport Agency said there was still a risk to road users if the other was opened.
Its regional performance manager, Karen Boyt, said it would take at least two months to install the protective mesh necessary to prevent further rockfall.
Contractors would be on site at all times managing traffic flow until then.