A dropout on State Highway 2 near Dannevirke is now being repaired in addition to existing ones on SH5 and SH2 north of Wairoa which formed after heavy rain.
A speed restriction of 50km/h has been in place since June 14 after slumping of the roadside shoulder on SH2, near the intersection with Otanga Rd.
The dropout is about 15m by 1.5m wide according to Waka Kotahi New Zealand Transport Agency.
Rob Service, Manawatū-Whanganui system manager, said the dropout was the result of a recent heavy and sustained rainfall event that affected several areas across the Central North Island region.
"We have been aware of this since the weather event and progressing with options to reinstate the shoulder.
"The primary cause is stormwater run-off and saturated ground conditions."
Service said repair work on the dropout starts this week and is expected to take two weeks depending on weather conditions.
"This section of SH2 will be reduced to 30km/h while crews are onsite working. Stop/go traffic management will be required to ensure worker safety and controlled movements of vehicles through the site."
Meanwhile, SH5 has been down to one lane since July 8 due to a large dropout between Tarawera and Te Haroto that had crept to within a metre of the edge of the lane.
The transport agency has established a construction site this week to return the road to two lanes, work that is expected to take about 10 weeks, dependent on weather and availability of materials, machinery and personnel.
The transport agency began surveying a big slip next to SH2 at Waikoau Hill, south of Tutira, last week after they reduced the major highway to one lane for safety reasons.
A smaller underslip has also developed on SH5, near the Te Pōhue golfcourse.
Hawke's Bay system manager Martin Colditz said the underslip was small and not currently threatening the road or road shoulder.
"Waka Kotahi has been monitoring a number of small underslips across the state highway network in Hawke's Bay following the March and April storm events earlier this year.
"We are in the early planning stages for investigation and design of a solution for this site and others."
Nick Leggett, chief executive of Ia Ara Aotearoa Transporting New Zealand, said investment into road maintenance from central government had not kept up with the real increasing costs.
"Over the last six years, the costs to maintain a kilometre of state highway have increased by well over 30 per cent, but the budget that goes from the government to NZTA to maintain that road hasn't increased by the same amount, so in real terms it has gone backwards.
"The chickens are coming home to roost as more and more extreme weather come and washes away infrastructure that has been poorly kept and hasn't had enough spent on it."
Leggett said feedback from truck drivers was that the current level of maintenance was unsafe and slowing the roads.
"The most frequent response we get, and I had one from the Hawke's Bay the other day, was 'I have never seen the roads in such a poor state of repair'."