The National-led Government's books could have been worse by hundreds of millions of dollars had it not decided to increase the petrol tax over the next three years.
Finance Minister Bill English says the tax increase will help the public finances get back to surplus by 2014-15.
However, he insists the petrol tax rises are intended to pay for road projects and not to keep the Government's promise to return to surplus.
He told Checkpoint the increase, together with a Cabinet decision not to cut ACC levies, will add more than $400 million to the Government's coffers.
Mr English says the Government cannot rule in or out any further tax increases.
Motorists will now have to spend 9 cents a litre more in petrol tax over the next three years to help build the Government's roads of national significance.
Without that money, its half-yearly economic and fiscal update could not have forecast a $66 million surplus in 2014-15.
Even then the Treasury warns that if things turn out worse than expected, the books won't get into surplus until at least 2017.
Overall, the Treasury is forecasting the economy to grow more slowly than it predicted in the Budget in May.
The unemployment rate is expected to fall but the current account deficit - the difference between what New Zealand spends and earns overseas - will get worse in the next five years.