The latest design changes to a proposed expressway north of Wellington have added $115 million to the cost of the $2.4 billion project.
The New Zealand Transport Agency has announced significant changes, including extra bridges and earthworks.
The agency's Wellington state highways manager Rod James says there's a chance the price could rise even further through the consenting process.
The Government has also acknowledged the total $11 billion cost of its Roads of National Significance programme could increase further.
Just before the the election in November, the then Transport Minister Steven Joyce said there were no substance to rumours of a major cost blow-out on the Kapiti Expressway.
A group of local residents opposed to the road, Save Kapiti, said the Government has chosen to hide the cost escalation until after the election.
Transport Minister Gerry Brownlee says rising costs will not deter the Government from its mission of fixing the country's ailing infrastructure.
"The only way you can be assured that you're not going to have to spend more is not spend anything," he said.
"It's unfortunate, but we've made no secret of our desire to improve the infrastructure throughout New Zealand, so we're not going to back off a policy for fear the cost may be rising a little too much."