Finance Minister Grant Robertson will unveil his fifth budget on Thursday in Wellington's parliament house.

The scene has been dramatically re-cast from his first years in office, when the budget surplus reached $NZ7.5 billion ($A6.8 billion), and net crown debt dipped below 20 per cent of GDP.

Defending the nation against COVID-19 has come at a fiscal cost.

After two bumper deficits, Treasury's most recent projection has net debt peaking about $NZ185 billion - or 48 per cent of the economy's annual output - in the forthcoming year.

"We have invested a huge amount in supporting New Zealanders," Mr Robertson said.

"We do need to return our fiscal settings to something like what they were."

The times call for more prudent spending, though the government is hell-bent on delivering two of its key election promises.

On Monday, Mr Robertson - in the absence of the COVID-hit prime minister - unveiled New Zealand's first emissions reductions plan.

The plan includes $NZ2.9 billion of measures to curb emissions, including a cash-for-clunkers scheme, decarbonisation handouts for business and large-scale native forestation.

Further measures will come on Thursday, along with a billion-dollar spend-up on health.

Citing the pandemic, the government is centralising the health system in the hope of improving service delivery in the regions.

"The budget is called A Secure Future because that security is provided by all of the investments that we make," Mr Robertson said.

"This is a budget that takes a careful balance. We do have to keep a lid on our debt. We do have to take a look again at the fiscal position of the government. But there's a lot of important issues to help deliver.

"It is about the pathway for a new normal. Not everything about the old normal was great."

Political battlelines have already been drawn over those big ticket items, with the opposition labelling the government's spending as wasteful.

National party leader Chris Luxon is likely to continue that attack no matter what is announced on Thursday, given Mr Robertson has set aside a larger-than-usual $NZ6 billion operating allowance for new spending.

The opposition's claim is that government spending is driving inflation - which sits at 6.9 per cent - a charge Mr Robertson denies.

"We will do what we can to address those underlying issues but many of them are global and beyond our control," he said, hinting at new spending that could curb inflation.

There are some things within our control ... things like supermarket competition. Making sure that we do get the labour force into New Zealand that we need."

In previous budgets, the Ardern government has also sought to buffer those less well off against cost of living rises, and it would not be a surprise to see welfare spending increased.

However, Ms Ardern will not be there to announce it, as she'll be on day five of a spell in isolation after catching COVID-19.

Australian Associated Press