Primary teachers and principals have voted for full day strike action instead of a previously planned three-hour strike.

The teachers' union NZEI has been seeking an immediate 16 percent pay rise and improved working conditions since pay talks with the Ministry of Education began in May.

NZEI union lead negotiator Louise Green says the decision to extend the strike action planned for 15 August sent a strong message to the government that it was time to fix the crisis in teaching.

A clear majority of NZEI's teacher and principal members groups voted in favour of a full day, she said, giving a "strong endorsement for collective action."

Louise says it's 24 years since educators had gone on strike and the action was not action being taken lightly.

Lead teacher negotiator Liam Rutherford says public opinion polling showed strong support for more taxpayer dollars being spent on education, including a significant pay increase for educators.

"The National Party's u-turn on teacher pay and new desire for smaller class sizes means there is now no political opposition to addressing the crisis in education.

"The members' decision to take industrial action shows the degree of frustration and conviction among teachers and principals."

Louise says it's crucial to attract and retain great teachers.

"Teachers and principals need to have time to teach, time to lead, and be valued for the professionals that we are. And all children need to get the support they need to thrive at school."

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