Photo: RNZ / Dom Thomas
The 27 kilometre stretch of motorway was supposed to cost $850 million but has now exceeded a budget of $1.25 billion.
The report, lead by an international expert reviewer Steve Richards and peer reviewed by Sir Michael Cullen and Lindsay Crossen has found serious flaws at the planning stage of the public-private partnership project, undermining its successful completion.
It was commissioned in August last year after the Transport Agency had to pay another $209.7million to get the project back on track after the Covid-19 lockdown.
Official documents show the negotiations went on for months and the contract could have been terminated completely.