He said the country's finances will be hard hit in fiscal years 2020 and 2021 before slowly recovering.
"The positive outlook on our ratings on New Zealand is supported by our expectation that deficits might not peak as high and large deficits might not be as prolonged as the budget indicates," he said.
Walker said he expected the New Zealand's economy to recover faster than the budget assumes, aided by the quicker easing of lock down restrictions than assumed in the budget.
"This will support revenue growth and could lower outflows from the Covid-19 response and recovery fund."
Most of the weakening in the fiscal forecasts was because of Covid-19-related spending and was not a substantial increase in ongoing expense measures, Walker said.
Westpac senior markets strategist Imre Speizer said the prospect of more bonds coming on stream caused some initial selling in the bond market.
The Reserve Bank said yesterday that it had almost doubled its bond buying programme to $60 billion so that it could support the market in face of the wall of new Government bonds coming on stream.
Speizer said the budget's measures could act to ease the pressure on the Reserve Bank if the Government is successful in stimulating growth.
"In the big picture, the budget on the whole is more stimulatory than expected, which means the Reserve Bank might not need to be as active as we thought yesterday," he said.
"They (the Government) have come out with a big bang. A big dollop of stimulus, so overall it's a positive," he said.
Fisher Funds fixed income portfolio manager David McLeish said the $165b bond programme was about $50 billion higher than he had expected.
The yield on the 2031 bonds rocketing up by 20 basis points to 0.73 per cent, reflecting the higher than expected supply. In bonds, yields rise when prices fall.
"Government spending is of course a shot in the arm for the economy and it needs it badly right now," he said.
"That's a good thing and it needed to be done," McLeish said.
"But there is no free lunch here."