An independent report on the Northland Expressway, is sowing doubt over the project's future

The NZIER has previously estimated the four-lane highway would eventually boost national GDP by 1.2 billion-dollars a year.

But researcher Simon Chapple, claims the company was under some incentive to reach conclusions supporting the project.

He says the decision to prioritise the highway is built on insufficient foundations.

NZIER says it rejects the claims, and say it was never under any pressure to reach a particular finding.