About one third of major roading projects ran over budget in recent years.
Information obtained by Radio New Zealand reveals 23 state highway and local roading projects costing more than $3 million, ran over budget by $121 million in the past five years.
The biggest blowout was $55 million for the Manukau extension in Auckland, which was originally forecast to cost $184.6 million.
The Transport Agency says overruns are not a surprise, because the way projects are funded allows for half of all projects to come in over budget.
But Labour and the Greens say the budget blowouts are unacceptably high.