Rock armouring was required where coastal erosion was occurring in 10 places along State Highway 1 at Katiki Beach, NZTA senior project manager Jason Forbes said.
Rock armouring — placing rocks along the shore to stop the sand being taken away and protecting the highway from erosion — was a "more viable alternative" to realigning the road, Mr Forbes said.
"The highway’s proximity to the railway line makes realigning the road a very expensive option," he said.
Erosion protection structures were put in place where the highway was threatened at Katiki Beach in the 1970s, and the stretch of coastal road had been monitored each year since 1994.
From 2008 to 2014, engineering services company Stantec had surveyed coastal regression and found, on average, the cliffs retreated almost 1m towards to the road every 10 years.
In 2017, three "high risk" sites were rock armoured to stop coastal erosion from undermining the highway.
"This new contract is to complete a further 10 sites identified as a high priority for protection work," he said.
"Most of Katiki Beach will eventually require some rock armouring, so coastal erosion doesn’t damage State Highway 1, a critical road freight and lifeline link between Dunedin and Christchurch."
Waitaki Mayor Gary Kircher said it was a "strategically significant" stretch of road in the district.
"It’s an appreciable amount of investment for NZTA ... it’s very pleasing to see that they are putting that money into it.
"And from an aesthetic point of view, it’s one of the few pieces of road for quite some way that you can actually drive alongside the ocean."
The council continued to monitor other "at risk" areas of coastal road in the district, such as Beach Rd, he said.
"Erosion has always been a problem, and it always will be."
Subject to weather and consenting requirements, rocks would start to be stockpiled at Katiki Beach early next year. The rock armouring work should be completed within a year, Mr Forbes said.
