It is hard to avoid a "yeah, right" reaction to the Budget's projections for two key measures of fiscal prudence.

They have the Government maintaining a sort of sweet spot over the next four years, running small operating surpluses while its gross debt remains steady around 17 per cent of gross domestic product.

It would not take much of an economic shock to turn surpluses of less than 1 per cent of GDP into deficits and send the debt ratio climbing.

The Budget itself sketches one such scenario, where the international environment turns uglier, the surpluses become deficits and debt rises to 22 per cent of GDP by 2012.

Rather than believe that sweet spot of tiny surpluses and steady debt-to-GDP can be maintained for years it is more realistic to assume that if the fiscal position is not improving it will be deteriorating.

Clearly, with economic growth at a standstill the kind of fiscal stimulus the Budget dispenses is appropriate and timely.

The danger is, though, that a temporary fillip to the demand side of the economy turns into an enduring deterioration in the fiscal position.

We have not seen fiscal deficits for many years, and neither have the rating agencies.

Given the parlous state of the country's external accounts, an extended period of deficits and rising debt could possibly put the country's credit rating at risk, raising the spectre of sequentially higher interest costs across the economy for years.

When this issue was raised with Finance Minister Michael Cullen on TV One's Agenda programme on Sunday he was sanguine: "I'm not deeply concerned about that."

He was able to point to a statement from Standard and Poor's put out immediately after the Budget, which reaffirmed its ratings.

"For the time being this Budget has maintained the Government's strong fiscal position," it said.

"While a significant weakening of fiscal policy would lead to a rating downgrade, this is unlikely under the current Government or under whichever government is formed after the election later this year."

But it also sounded a warning note.

"It is critical for New Zealand to maintain fiscal restraint given the uncertain global financial conditions and the imbalances in its own economy. The large current account deficit leaves New Zealand vulnerable to foreign investors losing confidence in its ability to meet its obligations and this could lead to a sharp reversal in capital flows."