The Prime Minister says the previous Government's budget didn't even cover the core basics - something she's slating as "hugely neglectful".
"There are things that as we had taken office we are seeing exist in other areas as well that we didn't foresee," Jacinda Ardern told The AM Show.
"What has stood out for me in the period we've come in, and there are actually examples… where just core basic things haven't been supported or funded for nine years."
We will not be able to address nine years of neglect in one Budget.
Ms Ardern dampened down expectations of the upcoming Budget, explaining the underfunding in "almost every portfolio" was much worse than she thought.
"We thought it would be bad. We didn't know it would be this bad," Ms Ardern said at her weekly post-Cabinet press conference.
"We will not be able to address nine years of neglect in one Budget."
Ms Ardern says there's a plan in place to reinvest in "core services", noting education and arts in particular.
She didn't give specific details of where the underfunding was but says she's "planning on sharing those in the House".
"The priorities that the last Government set were very much about the things that obviously were important to them.
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"But at the same time, we're letting a row down - some of the basic stuff we just needed to keep doing to make sure people got the services they expected."
"When you see the kind of decisions that Middlemore has made, I think people would consider that neglectful. Hugely neglectful."
Middlemore Hospital has become a focus after it was revealed the hospital has mould in the walls.
"Regardless of what [Jonathan Coleman] knew or not, what he did know is that the DHBs were really struggling; they were running deficits and that meant they were making trade-offs around what they were doing because they simply weren't funded properly."
But National's Finance spokesperson Amy Adams says the current Government is lining up its excuses now because it doesn't want to admit it didn't get the numbers right.
"There will be things that come up out of the blue. That is quite normal in Government. Under National, we managed that for nine years," she said.
Ms Adams said the previous Government faced unexpected costs from the likes of several earthquakes and the Rena disaster.
"We warned their numbers were too tight. They are starting the game of pointing fingers at everyone else."
Finance Minister Grant Robertson has laid out his Budget priorities: critical public services, economic development and supporting the regions, child poverty and homelessness, families and action on environmental challenges.
Watch the full interview with Jacinda Ardern above.
The AM Show with Duncan Garner, Amanda Gillies and Mark Richardson, weekdays 6-9am on RadioLIVE and streaming live to the rova app on Android and iPhone.