Secondary teachers are prepared to strike if the government does not improve its pay offer.
Members of the Post Primary Teachers Association (PPTA) will vote on industrial action next month, after rejecting the offer as "insulting".
Delegates at the union's conference in Wellington said secondary teachers were determined to win a better pay rise and were ready to strike if they had to.
They said the government's offer, which would have increased most teachers' pay by three percent a year for three years, was inadequate.
The union's decision raises the possibility of a double-whammy of strike action with the primary teachers union, the Educational Institute.
Its members have already rejected a similar pay offer and will vote soon on whether to hold a week of rolling one-day strikes.
New Zealand Educational Institute president Lynda Stuart told Morning Report this morning pay was a significant factor in negotiations but workload remained equally important and had not been addressed.
She acknowledged the scrapping of the National Standards assessment model had made a difference to workload, but much more action was needed to address unmanageable working conditions.
"What our teachers are saying up and down the country is they need more time, they need lower class sizes," she said.
"They're dealing with students coming into class with complex learning needs and challenging behavioural needs as well. They need the support in those areas and we don't see that in this offer at all.
"There was nothing in the offering about workload, teachers are being burnt out ... people are absolutely whacked at the end of term and that's because people are giving their all to the children of this country.
"We have to acknowledge that people are not going into teaching not just because of the pay, but because of the conditions. We have to address the issues."
Post Primary Teachers' Association representative Joe Hunter said giving teachers more time to teach and lowering average class size from 29 pupils to 25 was a fair and reasonable request and teachers were not prepared to wait any longer for a plan to resolve long-standing issues.
"After nine years of taking the hit ... now to think we are going to have to wait again while the ministry sets up working parties that may not have outcomes until 2020," she said.
"Meanwhile, we have teachers leaving the profession, 20 percent of teachers over 60 are leaving, and we have a drop of about 40 percent of people entering the profession, so there is a real crisis and the ministry needs to face facts."
She said the government offer didn't address fair pay or fair working conditions and the government needed to come back with a new offer and work out solutions.
Both union representatives said they were committed to dialogue with the government so that further strike action may be averted.