"Every school in the country will say they could use more money. So if it's about re-carving the existing pie, that's where the problem starts to emerge."
He said the funding of schools had slipped back in per-student terms during the last eight years and catching funding up needed to be a priority.
Education Minister Hekia Parata denied there had been underspending.
"We are putting over $11 billion of taxpayer money into education. It's more than foreign affairs, defence, transport and conservation," she told Q&A.
"But we need to be able to say that we're spending it as well as we could be in order to make the arguments for further future funding."
She said schools were the ones "on the spot" and were best to determine how to spend their money.
Teachers unions on Monday came out in strong opposition to global funding, saying it was a back door attempt to bring in "the failed experiment" of bulk funding.
The Ministry of Education denies it has anything to do with bulk funding and the ministry would continue to pay staff salaries.