State Highway One south of Huntly is being reduced to one lane for four nights a week for up to three months, to stop it collapsing into the Waikato River.
The Transport Agency began preparations last night to build a retaining wall along 40 metres of the river bank, about
1.25km north of the Taupiri bridge, requiring the closure of the highway's northbound lane.
Regional operations manager Trevor Fearnley said the initial plan was to close the lane from 6.30pm until 6.30am on Monday through to Thursday, although that might be altered to 7am until 7pm, depending on traffic flows.
A "stop-go" traffic control arrangement would be in place during the building of the new retaining wall on top of about 25 piles to be driven up to 15 metres through the riverbed.
Mr Fearnley said scouring by the Waikato River had begun to eat into the road's foundations.
But he said the road was not about to fall into the river, and the work was just "preventative".
"It's not a high risk at the moment but if we left it, during periods of prolonged or intense rain it would become a significant risk."
An average of about 20,000 vehicles use SH1 north of Taupiri each day, including 5000 in the 12 hours from 6.30pm.
