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.
Listen to more