The Transport Agency said the concerns were about:
• the strength of the viaduct deck panels under construction loads while the viaducts are being built
• the durability of the viaducts over their expected design life of 100 years
The concerns were raised with the NZTA earlier this year by a "former employee" of one of the project's consultants, engaged by the joint venture Northern Express Group, or NX2, that is building the highway.
NZTA project delivery senior manager Andrew Thackwray said two subsequent reviews had dispelled the concerns.
"The project confirms there are no outstanding or unaddressed safety issues relating to the design of the viaducts," he said in a statement.
RNZ has asked NZTA to release the claims and the reviews, but it will take weeks under the OIA.
As of last week, a third of the slabs had been put up at the Ōkahu viaduct; these will then have more concrete poured on top, which is when the real stresses go on the structure. Strong safety measures were in place for workers, the agency said.
Puhoi doesn't get panels till August.
Each 300m-long viaduct has hundreds of panels, typically 2.7m wide and up to 10m long, that sit on steel girders on pillars or piers, crossing between hills outside Puhoi, north of the Johnstone Hill Tunnels, and by the inlet.
The agency got a specialist bridge engineer to review the concerns, Thackwray said.
"The independent review found the design and construction was in accordance with standard international practice, and fit for purpose for safely carrying temporary loads during construction.
"The same system is used routinely in Australia and around the world.
"NX2 was approached separately by the same individual, and undertook a separate review which reached the same conclusions."
No changes had been made to the design, he said.
RNZ has asked if the slabs have been tested under real-life loads - with all the vibration you'd get from 50-tonne trucks running over them dozens of times an hour - and is awaiting a response.
"With respect to durability ... the design ... does not pose any long-term issues," the agency said.
The design had previously been peer-reviewed by independent engineers and then been run through design workshops that looked at safety.
It was confident that the joint venture had "robust quality controls".
However, NZTA has had quality control problems on major projects:
• Stretches of Transmission Gully are being ripped up and relaid
• More than $80m of ongoing damage to the surfaces of three highways in Kāpiti and Waikato; the agency has refused to disclose what investigations show is at fault in these roads
• Steel having to be replaced, triggering a redesign of bridges on the Waikato Expressway in 2016
The Puhoi-to-Warkworth highway is a public-private partnership or PPP.
A PPP at Transmission Gully has a long and intensifying history of over-runs and disputes that have pushed the costs up by nearly $200m.
As at Transmission Gully, delays caused by Covid-19 are pushing up costs at Puhoi, a ministerial briefing released to RNZ shows.
The Transport Agency has so far not detailed what these costs are.
The road builders NX2 comprises Fletcher Building and Spanish building giant Acciona, Higgins which is owned by Fletcher, and also HRL Morrison and Co and ACC on the financial side.
The next stage of the northern highway, Warkworth to Wellsford, is costed at well over $1 billion.
