A "high" to "extreme threat" of landslides has been identified at the $900m upgrade of State Highway 1 north of Auckland, and blamed on Covid-19.
The threat on the Pūhoi to Warkworth project is revealed in risk registers released to RNZ under the Official Information Act (OIA).
RNZ has asked the Transport Agency what that means but its statement did not address this.
One of the NX2 Project Risk Register states: "CJV is looking for solutions to mitigate amount of earthworks and get the main alignment slips out of the critical path. Action plan developed/ independent reviewer in causation / peer review ongoing....NX2 review legal protection."
"The risk of unstable soil slipping" had increased because earthworks were exposed to wet winter weather for longer than expected due to a construction hiatus forced by Covid-19, Waka Kotahi said.
The pandemic had before now already added almost $90m to the project's costs.
"Measures put in place to ... mitigate the risk of a landslip delaying construction include reviewing the design as more detailed geotechnical data becomes available, especially in high-risk areas," Waka Kotahi said.
However, the registers showed greater impacts than that: Not only is the design being reviewed - and peer reviewed - but also the project's insurance and legal protections, and the cause of the landslide threat, were being reviewed.
The agency did not address those matters with RNZ.
The two registers both listed two risks - slips, and "future" slips - and rated both as an "extreme threat".
The registers then listed various "controls", which they rated as "effective".
Each register then listed "treatment tasks", which were due at later dates.
Then, each register gauged what risk remained if the tasks were done, and as a result downgraded the threat from extreme to "high".
A third risk register, from April 2020, does not mention a landslide threat.
"The risk existed prior to August 2020, but at a lower grade which did not warrant inclusion in the formal risk register," the agency said.
This was the advice it got from the project's operating consortium, the Northern Express Group or NX2, which drew up the registers, Waka Kotahi told RNZ.
The 18.6km highway was on varying soil types and some were "unstable".
"NX2 advise that the risk level rating was increased due to the cessation of work on the project during the Covid lockdown period, as this left work undertaken on slopes and cuts during the dry season exposed over the wet winter period."
The highway is in hilly country north of the Johnstones Hill tunnels, country that requires two huge viaducts to pass through: The viaducts were themselves the subject of safety concerns raised by engineers earlier this year - and addressed fully, according to the agency.
The route was chosen a decade ago after investigations involving geotechnical and topography specialists.
Northland is known for soils that can be difficult to work on.
The registers are brief, use some cryptic language, and, crucially, two sections about the cause and consequence of the landslides risk have been blanked out from the OIA documents.
RNZ has asked the agency to reinstate that information.
The new SH1 has been promised as a "safer, more resilient route," Waka Kotahi said.
The registers note problems with earthworks which were meant to be completed this year.
The April register showed the construction joint venture of Fletcher and Spanish company Acciona was trying to speed the earthworks up, having begun last summer moving three million cubic metres of earth, but Covid intervened.
By August, the builders were seeking a solution to "mitigate the amount of earthworks".
RNZ is asking the agency if the landslide threat has pushed up the amount of earthworks needed.
Previously, earthwork problems have struck another major highway project, Transmission Gully, where a third more soil than expected had to be moved, or nine million cubic metres.
The Pūhoi-Warkworth highway's opening has already been delayed by about six months to mid-2022.
The cost has also been hiked by 10 percent this year to $877m mostly due to covering contractors' extra costs coping with Covid delays, on top of a 10 percent escalation previously since construction began in 2016.
In recent weeks, SH1 was being realigned at the Johnstones Hill tunnels to link to the new highway.
Dealing with the risk a landslide would delay a project was not unusual given the country's hills, the agency said.
It was confident the build was of "a high standard, with robust quality controls, which will ensure that the motorway and all of its structures are durable, safe and fit for purpose".
The registers lay out risks other than landslides, and the measures suggested, or taken, to deal with them.
The April register referred to incorrect traffic figures for the design of the highway, and in response a design review by NX2 and a legal opinion on liability, moving this from a "medium" to a "low" threat.
The September register referred to a dispute brewing involving the Auckland Transport Operations Centre, though it is not clear what.
The four-lane, two-way highway is being built to the west of the existing SH1 and will bypass Warkworth on the west.
The highway will be extended another 26km to Wellsford, though that is at least a decade away.
NX2 will operate the road for 25 years, under a public-private partnership that keeps the highway in public ownership.