Acting Prime Minister Winston Peters says a strike by primary school teachers and principals is not going to get them anywhere.

NZEI members will walk off the job on August 15.

"It's not going to get them anywhere because we as a Government have made serious promises. We're going to keep them but we never said we'd deliver them the whole lot in the first six months or in the first Budget after six months of government," Peters told Newshub today.

Peters questioned why the union did not take any action while the previous government was in power.

"Nine years later, here we are and the teachers want us to settle it all at the same time?"

Lead principal negotiator Louise Green said yesterday the decision to strike sent a strong message to the Government that it was time to fix the crisis in teaching.

"A clear majority of both member groups voted in favour of a full day, giving a strong endorsement for collective action," she said in a statement.

The full-day strike action replaces previously planned three-hour strikes.

"We had a clear message from members at the paid union meetings and from feedback and surveys.

"It is 24 years since educators have gone on strike and this is not an action we are taking lightly," she said.

Peters said negotiations were ongoing but the strike would have no impact.

"There's some sort of belief out there that it will help the negotiations by striking, well it won't, and I'm sorry to say that.

"We've got to carry on negotiating until we get to a resolution but striking and putting a whole lot of parents and children at enormous difficulty is not the best idea."

NZEI and the Ministry of Education agreed yesterday to enter mediation over collective agreement negotiations.

Any outcome of the mediation will be taken back to NZEI members for consideration.