The Government is defending the way it will fund the construction and maintenance of the Transmission Gully roading project, north of Wellington.
The Transport Agency announced on Monday a consortium will build the road but the Green Party says it will cost $3 billion, not $1 billion.
Transport Minister Gerry Brownlee says that figure does not take into account the maintenance the taxpayer will not pay for over 25 years.
He told Morning Report it is a choice between raising petrol taxes now, or spreading the cost over a number of years.
Mr Brownlee said the Cook Strait earthquake last August shows the time has come to provide the capital with a second access.
Greens transport spokeswoman Julie Anne Genter told Morning Report that taxpayers will pay three times more than they need to.
The Wellington Gateway Partnership comprises three construction companies with financial backing from the Bank of Tokyo-Mitsubishi and the Accident Compensation Corporation.
The Transport Agency will now negotiate final terms with the consortium before construction begins towards the end of next year.
Mr Brownlee said the negotiations should go smoothly with both parties clear on what is expected.