Ministers have been ordered to review their portfolios in preparation for the next Budget and identify at least 1 per cent of expenditure which is not in line with the Government's objectives, a leaked document reveals.

The document, leaked to the National Party, also shows what the priority areas for next year's budget will be.

These include supporting mental wellbeing for all New Zealanders, with a special focus on under 24s, and lifting Maori and Pacific incomes, skills and opportunities.

The instructions to ministers said: "Each portfolio minister will be required to undertake a review within their portfolio(s) to identify at least one per cent of expenditure within each portfolio's baseline that is not in line with the Government's objectives, or is of the lowest priority."

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Although it's not unusual for spending to be reprioritised – $1.4 billion was reprioritised over five years in this year's Budget – National Finance Spokeswoman Amy Adams said the instructions read as if ministers were being asked to cut spending in their portfolios.

"I think what it shows is that the Government finances are incredibly tight and makes a mockery of this Government saying many areas are underfunded when the first thing they want to do is find ways to cut expenditure."

In a statement to the Herald, Finance Minister Grant Robertson said: "As I said in the House last week, reprioritisation is an ongoing process".

"To respect the confidentiality of the Budget process, I am not going to comment on any specifics for Budget 2019 as the Budget process is underway."

Last week he confirmed there would be five budget priorities and one would be in relation to mental health.

He is likely to publicly release these at Thursday's Budget Policy Statement (BPS).

As well as supporting mental health and lifting Maori and Pacific incomes, other priorities – according to the document – include:

• Creating opportunities for productive businesses, regions, iwi and others to transition to a sustainable and low-emissions economy

• Supporting a thriving nation in the digital age through innovation, social and economic opportunities

• Reducing child poverty and improving child wellbeing, including addressing family violence

Adams said these priority areas were all "high level and reasonably generic" and don't give much detail as to how the Government would make its flagged "transformative" change.

"This supports my view that the Wellbeing Budget, other than the branding and the platitudes, is not going to be terribly different from what New Zealanders have seen in previous years."

In a statement, Robertson said: "Budget priorities will be announced as part of the Budget Policy Statement on Thursday."

Robertson said previously said next year's budget – dubbed the Wellbeing Budget – would broaden the focus beyond economic and fiscal policy.

As well as the BPS being unveiled, the Half Yearly Economic and Fiscal Update (HYEFU) will be released too.

HYEFU shows the state of the Government's books, detailing if the Government has a surplus, what its level of debt is and various other fiscal and economic measures.

When the books were opened up during this year's budget, it showed a $7.3 billion surplus was expected by 2021/22 and net debt to fall to 19.1 per cent by the same year.