The newly appointed finance minister has painted a dismal fiscal picture ahead of the incoming mini budget but promised to find ways to grow the economy and to get the books balanced again.

The Government announced the mini budget will land on December 20 and outline actions the new regime is taking around fiscal discipline.

Minister Nicola Willis told the Mike Hosking Breakfast that she believes the previous Government left fiscal booby traps, and she is working through the mess systematically.

"We knew the spending was out of control and the books were in bad shape - but what's becoming clearer is the scale of the financial messes that Labour left behind," she said.

"We're just trying to be open with the public about the scale of the challenges we're facing."

Willis said the incoming Government is facing financial blow-outs on "a number of fronts", including numerous major projects, some of which the finance minister claimed have more than tripled in cost without any plan to fund the extra expenditure.

Some ongoing programmes that the public will expect to continue haven’t been funded well, she said, which leaves the new Government to find money to afford them.

One was the school lunch programme.

"As it is, we'll be looking for efficiencies in that programme because it will expand in pricing into the future and there aren't the dollars for it in future years," she said.

"I have to look taxpayers in the eye and say every single one of their dollars being used in the school lunch programme is being used well, that it's not being wasted, and I don't think Labour have been doing that exercise."

She said the incoming Minister of Education will need to review the lunch programme and reduce any waste.

Hosking noted the latest financial numbers released over the weekend, which showed Government expenditure jumped from $75b to $123b, and a spike in Government debt.

Current debt sits at $181b. It was only $62b when Labour took charge in 2017.

"There definitely will be areas of wastage," said Willis.

"Now I'm behind the curtain, on the other side, I'm working carefully with our Treasury officials to put together a programme to cut out the waste, cut out the excessive spending and needless bureaucracy, and ensure better value for money."

Willis was quick to defend the Treasury when it came to questions over its performance in recent years.

She said there was a high likelihood it was firing warning shots when Labour was making decisions around the books - but "they have a big job now to help me tidy it up".

The next steps looked like the Treasury fronting up to some of the "poor practices" used in the past and, in some cases, Willis said, rules needed to be changed to ensure it doesn't happen again.

"I don't think anyone should be left with these financial booby traps going on."

When Willis was asked whether she worried about the future of the economy and if forecasts were reliable enough to trust, she again backed the Treasury and claimed forecasts were difficult.

But her message was one of reassurance that while there was cause for concern, she believed her Government was capable of getting the economy back onto a track of growth.

"Yes - there's lots to be worried about in the global economy and there are forces that are coming our way, but also, it's a new day," she said.

"We've got a new Government, a Government that backs business and entrepreneurs and wants people to invest, that wants small businesses to be employing people and having a better go, we want to get red tape out of the way - we are on the side of growth."