James Shaw says he'll learn from his mistake after an 'error of judgement' over funding for Taranaki's Green School.

The Green Party co-leader rolled out a string of apologies over signing off on almost $12 million for the privately run school.

The decision drew objections from other schools and within his own party, which doesn't support private schools.

Shaw - who's also the Associate Finance Minister - says as a politician, admitting you were wrong is one of the hardest things to do.

"Becoming a Minister means being willing to question your decisions in public, and, if necessary, to correct them, and that's what I'm doing." 

He says his personal view is that the support should come in the form of a loan rather than a grant. 

Shaw's apology has raised one opponent's respect for him, but it doesn't make the situation better.

Marfell Community School principal Kealy Warren penned an open letter calling the funding totally unacceptable.

She told Heather du Plessis-Allan that there is only one way of sorting it out.

"That the Green School do not get the money, and that state schools get their buildings fixed."

Warren calls the event "disgusting and horrible", but is glad that the suggestion of making it a loan instead is out there. 

She says that she would prefer if the school just gave the money back.