Secondary school teachers will strike next month if an agreement on pay and working conditions cannot be reached.
The Post-Primary Teachers Association made the announcement today at a conference in Rotorua.
The union said teachers were running out of patience and would strike for a full day on 3 April.
Its vice president Melanie Webber said negotiations had been ongoing for months but had so far been fruitless.
"Issues are not being answered. We've been working now for nearly seven months and we're just not getting to where we need to be to be sure we have quality teachers and so we're being forced to take this action.
"There have been some offers but they're not sufficient to resolve the issues we have around workload and the corrections that we need. We've had a 40 percent drop off in the number of people training to be teachers, we're really worried that we're not going to have teachers in front of our students."
Ms Webber said teachers faced excessive workloads and there was little incentive to retain those already working and to attract new people to the job.
A pay increase was part of the solution, she said.
"We're looking at a correction that would bring us back in line to where we were probably in the early 2000s. There's been a significant drop from being 1.7 times the median wage down to 1.4 times the median wage and we're seeing that in the shortages we're seeing that in the classrooms where people are simply not wanting to take on what is one of the most glorious jobs we've got."
The government also needed to seriously address the teacher shortage and recent moves to bring in overseas teachers had done nothing to alleviate the issues, she said.
"We're still hugely short of teachers, it's not resolving the issues that we've got."
Union president Jack Boyle said the offers that had been laid out did not "touch the sides" of the issues.
"We hope to receive a realistic offer from the government for teachers to consider on 3 April, but if we don't, we will use the day to publicly protest the lack of progress."
In November last year union members turned down the latest government offer and committed to strike action in Term One of 2019.
Ms Webber could not confirm if the primary-school union, NZEI, would strike on the same day.
"NZEI is having their meetings later in the month they're at a similar point to us - their collective agreement's about to expire. I can't speak to what they're doing but certainly we stand with our cousins in primary."
Last year the heads of both unions [https://www.radionz.co.nz/news/national/375767/primary-and-secondary-teachers-may-have-combined-campaign-in-2019
met to discuss] potential joint strike action.
Principal of Glendowie College Richard Dykes said there was a serious staffing shortage and teaching at a public school just wasn't attractive.
"I knew of one school who lost a teacher to an independent school because they were outbid by $18,000. For a government who talks about equity, they've got to front up and say we're prepared to address that," he said.
The Ministry of Education said while it acknowledged the high demand for teachers, it said it had enough.
Spokesperson Ellen MacGregor-Reid also said it would continue negotiations within the $496 million package available.
Work was ongoing to address the key issues raised by the union, she said.
"The government has an extensive work programme underway that will help address workload concerns. A joint taskforce (including the PPTA) is identifying compliance-related administrative tasks for streamlining and the NCEA review has the potential to help including reducing the workload associated with internal moderation, which would be effective this year."
Ms MacGregor-Reid said efforts were also being made to encourage people to take up the profession and the drop-off rate was doing a U-turn.
"The decline in ITE (Initial Teacher Education) enrolments have been slowing, and our most recent information suggests that primary and secondary enrolments have increased this year. There have also been 350 secondary teachers who have enrolled in the Government-paid for, Teacher Education Refresh Programme."
The PPTA and the Ministry would meet on Thursday and Friday next week to continue talks.
Education Minister Chris Hipkins said he understood teachers' frustrations.
"Their demands were ignored for nine years under National and the number of trainee teachers was allowed to drop sharply.
"The government's nearly $500 million offer for secondary teachers alone offer will see the majority of their salaries rise by $10,000, and we are trying to work constuctively with them on workload issues.
"We have also put short term measures in place to ensure there are enough teachers on hand to cover any shortages. We have already invested $40 million to grow teacher supply to try to catch up with the shortages we inherited, and strongly encourage any schools who have vacancies to pick up the phone to the Education Ministry or recruitment agencies."