The Prime Minister says it is simply untrue the Government had committed all its money and now can't afford its promises.

Jacinda Ardern signalled next month's budget might not contain any treats because under-funding in areas such as health is larger than expected.

She called it the "rebuilding budget" for core public services.

Ardern says Labour set aside more money than National had for unforeseen spending - $2.6 billion as compared to $1.7b.

She told Mike Hosking the purse strings are tight because they're choosing to fund departments properly.

"I think the difference here is that we just have different priorities. For instance, the last government, they choose to invest at a level in health where the DHBs were running deficits."

However, Ardern does admit some of its policies might not happen on time because they'll have to fix things such as hospitals with sewage in the walls.

A major promise is to run a surplus and pay debt down to 20 percent of GDP by 2022.

Ardern says they'll still be able to do that despite a few bumps in the road.

"That means no tax cuts, and yes to investment in health and education. You can do both, it just means some of the things we've got to do are going to take us a little longer."

LISTEN ABOVE  AS JACINDA ARDERN SPEAKS WITH MIKE HOSKING

Despite the books being in a much worse state than expected, Finance Minister Grant Robertson says that won't mean an extension to his promised debt repayment track.

He's also indicated that expectations across the board need to be lowered.

But Mr Robertson says they'll work to fill the gap without breaking their promise of lowering debt to 20 percent of GDP by 2022.

"We believe we can make these critical investment and still stick to the rules that we've got, and you'll see outcomes of that soon."

However, National is predicting there will be broken promises in the Budget.

National's finance spokesperson Amy Adams says one of three things will happen.

"Either we'll be borrowing a lot more, and/or paying more in taxes, or they'll be dialling back on their promises. They need to be honest with New Zealand about which of those, or which combination of those, it's going to be."