A glitch has been found in a Treasury costing tool.
National says it could derail this year's lauded Budget, but the Government - and Treasury - rejects that.
For 2019's Budget, dubbed the 'Wellbeing budget', every dollar spent must be proven to improve the lives of Kiwis.
To measure that, Government agencies use a Treasury tool called CBAx to pitch for Budget money.
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CBAx operates through a spreadsheet. It assigns a monetary value to everything - from the cost to Government for a day of hospice care ($722 per day), to things like the personal benefit making a new friend, which is worth $592.
But there's a problem, any small change to the spreadsheet throws the numbers way out of whack.
Newshub reordered the spreadsheet, and the money saved from someone avoiding diabetes went from $3916 per year up to $45,209 a year. Another reorder spat out a cost to the Government of more than $4000.
National's Finance spokeswoman Amy Adams told Newshub it had the potential to force Grant Robertson to redo his entire budget.
"It undermines all the work that's been done so far, and I think does force them to go right back to square one."
But Treasury Deputy Secretary Struan Little told Newshub that Treasury instructions specify that users should not alter the spreadsheet.
He said even if altered, Treasury experts would pick it up and fix the issue with agencies' proposals before it led to any problems with the Budget.