Primary teachers and principals have voted for full day strike action instead of a previously planned three-hour strike.
Watch NZEI lead negotiator Louise Green on Morning Report:
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.
Lead negotiator Louise Green said 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."
Ms Green said it would be up to boards of trustees to decide whether to open schools during the strike.
"We know this is an inconvenience for parents, but we also know that our parents want the best education for their children," she told Morning Report.
"We think we have parents' support."
Ms Green said it was 24 years since educators had gone on strike and the action was not being taken lightly.
"We know that there are 40 percent fewer people entering teacher training at a time when we have a crisis, when we are really struggling to get teachers into our schools."
Lead teacher negotiator Liam Rutherford said 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," he said.
"The members' decision to take industrial action shows the degree of frustration and conviction among teachers and principals."
NZEI and the Ministry of Education agreed yesterday to enter mediation over collective agreement negotiations.
Education Minister Chris Hipkins said he hoped mediation would head off the full-day strike.
Mr Hipkins said teachers wanted to be paid more but also wanted working conditions and school resourcing addressed.
"So we've got a balancing act to achieve here. I think one step at a time. We have some mediation starting this week and we'll be going into that in good faith."
Ms Green says the outcome of mediation will be taken back to NZEI members for their consideration on 15 August before any decisions are made on future action.