Ardern said this was still affordable because her party would scrap National's $2 billion tax cuts and bring down debt two years later than National's target of 2021.
"Certainty there was some anticipation that [the surpluses] would be better, but that does not interrupt our plans."
Despite the softer economic outlook, Ardern would still not rule out bringing forward the party's free tertiary education policy, which would not be fully implemented until 2025.
Finance Minister Steven Joyce said the fiscal update showed that spare cash would be "tight" until 2020. For that reason, a second round of tax cuts would be off the table until 2020 unless economic conditions drastically changed.
"There is very limited room for additional expenditure of any type until then," he said.
Even in 2020, the party would only go ahead with new tax breaks for families if it could do so without borrowing or reducing its spend on health, education and infrastructure.
Prime Minister Bill English said the Treasury results showed that tax hikes were not needed to deliver on core public services and lift incomes.
Treasury predicted 216,000 more jobs would be created by 2021 and the average wage would rise by $6500.
English said Labour would not be able to implement its policies without borrowing or raising taxes.
That was rejected by Ardern, who confirmed that Labour would neither raise income tax nor ask a planned working group to consider it if Labour was in Government.
"We are not campaigning on an increase in taxes for personal income, and it is not part of our plans," she said.
The party will outline its economic plan next week, which appears likely to include a tax on tourists. The party has already promised a tax on freshwater.
Green Party co-leader James Shaw said short-term growth and bigger surpluses gave the Government room to spend on environmental and social policies, rather than tax cuts.