The strike - believed to be the largest industrial action by New Zealand teachers - is strategically timed to happen on the eve of the Government's May 30 "Wellbeing Budget".
It is aimed at persuading ministers to spend more on boosting teachers' pay and conditions than the $1.2billion over four years that has already been offered.
Otago Secondary Principals' Association secretary Gavin Kidd said the disruption that would be caused by the strike showed how determined teachers were to improve their working conditions.
"Retaining quality staff is essential and the working conditions are certainly putting good teachers off staying in the profession.
"It certainly expresses the determination of the teachers for things to improve."
Otago Primary Principals' Association chairman Chris McKinlay said teachers did not believe Education Minister Chris Hipkins and the Prime Minister when they said there was no more money, given the spending announcements that had been made over the past 12 months.
"The primary sector feels as though the Government hasn't given enough, and they are going to stand their ground until they get a settlement that better meets their needs."
New Zealand Principals' Federation president and Bathgate Park School principal Whetu Cormick said he was not surprised the New Zealand Educational Institute (NZEI) and the Post Primary Teachers' Association (PPTA) had come together to strike.
"The coming together of the two unions is significant. It's unusual and it shows the depth of the feeling within the education sector."
However, Ministry of Education secretary for education Iona Holsted said the strike action did not offer a solution.
"Instead, it will cause disruption to the learning of many thousands of children and young people, as well as causing significant inconvenience for parents and families.
"Those with primary-aged children have already had to manage through two previous strikes."
She said the ministry would apply to the Employment Relations Authority for an urgent return to facilitated bargaining with NZEI to reach a settlement for primary principals and teachers, and would also start the process with the PPTA for secondary teachers.
"We are seeking facilitated bargaining because it is the strongest form of assistance available to the parties involved, to reach a negotiated settlement.
"Throughout our ongoing pay discussions we have been willing to discuss options for the combined $1.2billion settlement.
"We have also invited both unions to talk about how we can address their time and other concerns outside of the pay talks."
She said to date, the Government's investment in education included 600 new learning support co-ordinator roles as part of $500million being spent on learning support.
"These co-ordinators will be trained teachers and will support teachers who are working with children and young people with additional learning needs.
"The latest data shows that more people are training to be teachers, more teachers are entering the workforce and more teachers are staying in the profession.
"The teaching workforce is stable and growing, with over 1000 more joining our current 70,000 primary and secondary teachers in 2018."
Both primary and secondary teachers have been offered pay rises of 3% a year for three years, plus an extra top grade that would give many teachers effective pay rises of 12% over three years.
While the NZEI and PPTA are holding out for more staffing and more classroom release time to reduce heavy workloads, Education Minister Chris Hipkins has repeatedly said the Government would not offer more than the $1.2billion it has offered so far, and was only willing to talk about how that sum could be spent.
National Party education spokeswoman Nikki Kaye said it was crucial the Government provide a "circuit breaker" to resolve the issue.
"Labour created huge expectations with the sector by overpromising.
"Children's learning is suffering and people's lives are being disrupted.
"This standoff needs to be resolved. Labour needs to prioritise teachers to resolve the dispute."
