Education Minister Hekia Parata has fired back at accusations from the country's two biggest education unions that she has used "alternative facts" on school funding.

In an unusual move, the NZEI and PPTA called a press conference at Wellington Girls' College to challenge the outgoing Education Minister to explain herself.

Parata has in recent media interviews said funding for schooling has increased by 35 per cent.

At the press conference, Paul Goulter, NZEI national secretary, said the per pupil spending on schooling increased in real terms by only 2.3 per cent from 2009 to the end of 2015.

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That figure had been confirmed by Infometrics, he said.

"This Government now has to get real and address this in this Budget and lift the resourcing that our schools urgently need - not run around with alternative facts about a 35 per cent increase in funding."

Melanie Webber, PPTA vice president, said Parata's comments on funding weren't supported by what was happening in schools.

"It's certainly not supported in what we see on the ground. You can do as much mathematical skull-duggery as you like, it doesn't change what's happening."

Parata pointed to figures that showed the Vote Education budget had increased 35 per cent from the actual spend in 2008/09, to the budgeted amount for 2016/17.

Vote Education includes early childhood education (ECE) funding.

Operational grant funding to schools has gone up by 38 per cent since 2008. Student numbers have increased by 3.6 per cent in that time.

"Funding has increased at a faster rate than inflation. From 2010 to 2016, the cumulative increase in CPI inflation was 10.9 per cent. The cumulative increase in the operational grant funding rates over this period was 16.8 per cent," Parata said.

The unions have recently clashed with Parata over a Government overhaul of school funding, particularly a "global budget" proposal that was labelled a return to bulk funding by both the PPTA and NZEI.

Parata dropped that proposal but the Government's Education Funding Review is ongoing.

It is likely to see the decile funding system scrapped and replaced with targeted funding to children deemed at risk.

Any replacement system will be in place by 2019 at the earliest.

Parata will retire from politics this year and is likely to be replaced as Education Minister by Nikki Kaye in May. Kaye is currently the Associate Education Minister.

Goulter said today's press conference did not signal a deterioration in the relationship between his union, NZEI, and the Government.