But leaks, allegations of hacking and calls for the resignation of the head of the country's Treasury department threatened to steal the spotlight on Thursday.

The Ardern government on Thursday handed down what it's labelled its "Wellbeing Budget" - its second go at the books and one that has drawn attention from the likes of the Organisation for Economic Cooperation and Development.

"We are measuring our country's success differently," Finance Minister Grant Robertson said.

"We are not just relying on gross domestic product, but also how we are improving the wellbeing of our people, protecting the environment and strengthening of our communities."

While the usual financial measures were still there, the plan was laid out across key social welfare areas the government says need improvement based on international standards, and which it expects to regularly track in the future, such as child poverty.

In practical terms, a record $NZ1.9 billion ($A1.79 billion) of new spending over four years has been put towards mental heath programs and suicide reduction, and $NZ1.1 billion towards child welfare initiatives.

About $NZ5.5 million has also been set aside for mental health support for Christchurch after the March 15 mosque shootings in the city.

Another $NZ50 million is going towards boosting intelligence agencies, which have come under fire since the attack, while $NZ150 million has provisionally been allocated for gun buyback announced after the shootings.

But the opposition has labelled the budget a branding exercise with little change.

Despite the wellbeing focus, ministers have still found $NZ1 billion for investment in rail in a surprise announcement, under the banner of transforming the economy.

The defence force has been promised nearly $NZ470 million in new operational spending over four years, on top of $NZ1.7 billion earlier announced for new planes.

While expectations for New Zealand's economy have slowed over the past year, the national books are predicted to stay in the black to the end of the forecast period, backing a four-year spending allowance that increased from NZ$2.4 billion in the previous budget to NZ$3.8 billion on Thursday.

The government ran an operating surplus of $NZ3.5 billion over the past year and expects that to rise to $NZ6.1 billion by 2022/23.

But the government has warned of headwinds and GDP growth for the year to June dipped has to 2.4 per cent. It is forecast to rebound to 3 per cent in 2019/20 based on increased government spending.

While the budget immediately won praise from social agencies on Thursday, economists this week told the AAP that the approach taken had been less stringent than welfare rules already employed by some European countries.

"Every government has done work on health, education and policing ... I think it's more focused, and the rhetoric has been cleverer," Victoria University wellbeing and public policy professor Arthur Grimes said.

However, just hours before Thursday's launch, New Zealand's Treasury admitted an error on its website allowed highly sensitive budget information to be accessed earlier by the public, having earlier suggested there had been an stolen.

The department this week said it had been "deliberately and systematically hacked", after the country's centre-right National Party released details from the Labour-led administration's upcoming national accounts.

But on Thursday officials revealed a problem with a test website had actually meant some details from the budget could be found using a search on Treasury's website.

Hours before the budget launch, opposition leader Simon Bridges held a press conference calling for the resignation of Finance Minister Grant Robertson and Treasury head Gabriel Makhlouf for what he said had been deception and smears.

Robertson and Prime Minister Jacinda Ardern both declined to discuss the saga on Thursday.

Australian Associated Press