Continuing controversy around the announcement dubbed Bridges' 10 Bridges includes the national road funding agency saying the Government might have to find alternative ways to pay for the project.
The 10 Bridges scheme revealed during the Northland byelection campaign by Transport Minister Simon Bridges might not meet New Zealand Transport Agency (NZTA) funding criteria.
Conservationists are alarmed that widening one of the bridges could destroy two landmark, protected kauri trees in the Waipoua Forest.
But not all 10 bridges on Northland's state highways are currently in NZTA's sights, the agency's Auckland and Northland Regional Director Ernst Zollner said.
NZTA was informed about the announcement the day before the Government pledged to spend up to $69 million widening the bridges over the next six years, he said.
The scheme has to be assessed by NZTA to see if it fitted the National Land Transport Fund's (NLTF) cost-benefit and funding criteria. If it passed the test, it could make the 2018 funding round, Mr Zollner said.
"Alternatively, the Government may decide on a possible investment approach outside of the NLTF."
The NZTA planning and investment process applies to all investments from the NLTF, "and there can be no exceptions", Mr Zollner said. One of the bridges, the "Darby and Joan" bridge flanked by two mature kauri of the same names in the Waipoua Forest, is on a stretch of highway that was rebuilt and sealed in the 1990s. A purpose designed, one-lane bridge between the trees raised the pavement above the root system with the job winning environmental and road industry awards for its engineering and conservation values.
Northland Forest and Bird chairwoman Kristi Henare said the group had fielded calls from people concerned the bridge plan might compromise the world class environmental heritage site.
"We've already got this huge concern about kauri dieback disease and the collapsing forests, and here you've got two healthy kauri trees that might have to go to replace a bridge that doesn't need replacing," Ms Henare said.
Mr Zollner said the Darby and Joan bridge is being investigated for strengthening and other resilience improvements.
Four of the 10 one-lane bridges are already included in a local government draft 2015 Regional Land Transport Plan: Matakohe (Hardies and Andersons), Taipa and Kaeo.