The Green Party is questioning the Government's revenue forecasts on petrol and diesel taxes.
The Government says it expects to collect $33.9 billion over the next 10 years - $200 million more than previously forecast.
The Transport Ministry says its forecast is partly based on cheaper oil prices, but the Green Party's transport spokesperson says that is a stretch.
Julie Anne Genter believes it is time to move away from the current forecast models, as they are often wrong.
But the ministry says it is continually assessing the accuracy of its forecasts by tracking actual revenue against forecast revenue.
Transport Minister Gerry Brownlee has been unavailable for comment.