There will soon be both a high road and a low road on a stretch of SH4 in the Parapara hills.
A lower, temporary road is being built along the line of the former highway. While it is in use the contractors, Higgins, will be able to do some long-term repair on a steep cutting where papa rock is slipping downhill.
The problem cutting is between Otoko Pā and Kakatahi School, and there may be hold-ups for travellers when the repair work begins.
It should be finished by March next year, NZ Transport Agency systems manager Ross l'Anson said.
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The 75km stretch of highway between Whanganui and Raetihi is vital for Waimarino and Whanganui people. It's also one of just three exit points if Whanganui is cut off by weather or other emergencies.
It has to wind its way through steep hills with soft and unstable sandstone and mudstone (papa) soils.
It was badly damaged by heavy rain in June 2015, and l'Anson said $45 million would have been spent on the resulting repairs by the end of 2019.
Keeping the fragile road open costs about $9m a year.