Unstable ground around Dome Valley north of Auckland threatens to rip the wheels off the Government's "road of national significance" from Puhoi to Wellsford.
Transport Agency northern highways manager Tommy Parker has admitted concerns about the feasibility of completing the road as a four-lane expressway past Warkworth.
"There are alternatives outside the Dome, but you are still going the other side of the valley and we have a lot of ground movement," he told the Herald.
"Every time you put a spade in the ground up there you've got to put in retaining structures, or tunnels or something.
"The level of ground movement is more than we had anticipated, which makes huge problems and huge costs."
Mr Parker indicated that even the first 18km stage of the project, between Puhoi and Warkworth, posed geological challenges.
But engineers were concentrating on that section, making him optimistic of confirmation in a few weeks towards a target construction start in 2014.
"It's mainly the geology and engineering issues that are the most problematic up there, but we are still hopeful of confirming the route."
Meeting a target for starting the 20km Warkworth-Wellsford leg in 2016 would be tougher.
Asked how it could be completed within a $670 million estimate, when the shorter Puhoi-Warkworth leg would cost $980 million, Mr Parker said it depended to what standard it was built. That included whether it was built with four lanes or two.
Of a suggestion that the agency should settle for providing safety upgrades to the existing State Highway 1 through Dome Valley, he said: "That is an option that is being considered along with the others."
But Transport Minister Steven Joyce insisted later that nothing short of a four-lane road would do.
"I don't have a strong view as to whether the road between Warkworth and Wellsford is built on the current route or a new alignment," he said.
"But it is crucial for it to be built to roads of national significance standard - a four-lane divided expressway - and that remains the Government's expectation."
The safety upgrade idea was raised last year by the Campaign for Better Transport, which suggested choosing between two sets of improvements to the Puhoi to Wellsford route costing $160 million and $320 million. Both included a $50 million bypass of Warkworth and safety upgrades including extensive median barriers for Dome Valley and other black spots on a route where 44 people have died since 2000.
The more expensive would include a realignment of the difficult Schedewys Hill north of Puhoi.
Campaign convener Cameron Pitches noted a cash-flow problem which the Transport Agency says has forced it to delay $15 million of improvements to Warkworth's Hill St intersection with SHI, a traffic bottleneck, by at least 12 months.
"For us the obvious solution is instead of spending $1 billion on a new toll road to Warkworth, let's just look at working with what we've got and making that safer," he said.
Auckland Council transport chairman Mike Lee said many more people would be killed unless the agency switched resources from planning what he calls Mr Joyce's "holiday highway" to repairing design weaknesses on the existing "killer" road.
But fellow councillor Penny Webster, a former Rodney District mayor, said a four-lane highway was desperately needed through Dome Valley even if it required tunnels.
Mr Joyce accused "Mr Lee and his mates" of trying to think up excuses not to build the road.
It remained crucial for the economic development of Northland and the safety of one of the country's busiest rural highways "that we have a clear strategic plan to upgrade it to a standard that befits the level of cars and freight that it carries".
Estimated cost: $1.65 billion.
* Puhoi to Warkworth: 18km, $980 million.
* Warkworth to Wellsford: 20km, $670 million.