Manawatū-Whanganui system manager for maintenance and operations, Rob Service, said contractors had done their best to keep both lanes operating since June by topping up the road surface.
"To date, about 60 tonnes of asphalt have been added.
"However, we recently identified further slipping after more wet weather and decided to reduce the road to one lane. People were also not complying with the reduced speed limit that was in place for safety reasons."
"We have identified issues at the site with instability of fill material from an old road realignment. Reducing the road to one lane has now moved traffic away from the unstable ground where there is significant damage to the road."
Crews were monitoring the site around the clock to ensure the safety of road users.
Waka Kotahi were unable to give a timeframe on the work, but appreciated motorists for their patience and understanding.
Service said they were aware there would be long traffic queues and delays this weekend and that it was likely to occur again on Labour weekend.
"We are working to confirm long-term repair solutions to address the instability and return the road to two lanes as soon as possible - however, the traffic lights will likely remain in place for some time."
Further work was also planned for SH2 in the Tararua district, with maintenance works taking place over the summer construction season.
Several sections of the highway would be resurfaced and a number of full road renewal sites were planned between Woodville and Norsewood, Service said.
"We acknowledge the disruption our works can have to people's journeys, and want to reassure the public that we are doing our best to minimise the impacts as much as possible, while prioritising the safety of everyone involved."
Those planning trips between Norsewood and Woodville were advised to check the Waka Kotahi Journey Planner website.
"Our Journey Planner is the most up-to-date source of information for work taking place on the State Highway network. Keep yourself informed and plan your journey to minimise any potential disruption or delays."