More than 40 per cent of early learning centres could have at least one teacher resign as result of not wanting to be vaccinated against Covid, a group representing about half the sector says.
However, a group representing secondary school teachers says that by contrast only a "tiny amount" high school staff are refusing jabs.
It comes as all staff at schools and early learning centres had to receive their first Pfizer vaccine dose by Monday by order of the Government.
Those who choosing not to be jabbed can no longer enter school or learning centre grounds and are likely to lose their jobs as a result.
Simon Laube, chief executive of the Early Childhood Council lobby representing early learning centres, said a survey of about 300 members found 41 per cent expected to lose at least one teacher.
"This is a huge issue for centres who must maintain teacher/child ratios or send children home," he said.
"(And) 33 per cent of members surveyed say they'll lose one or two teachers, and 10 per cent say they'll lose between three and five."
The Government mandate came into force as Auckland primary school and younger high school in Auckland also went back into the classrooms this week for the first time since the city went into its Delta lockdown in August.
The no jab, no job mandate - which is supported by most in the sector - was brought in with the goal of protecting children, who cannot yet be vaccinated, and so rely on adults to slow the spread of Covid-19.
Health officials say vaccinated people are less likely to catch and thus spread the virus - and are also thought to be less likely to infect others although more research is needed.
But some school and early learning staff have refused to get the jab, based on a variety of reasons - often based on incorrect information that the vaccines are not safe.
That has led to fears schools and early learning centres will be short staffed.
Laube said that will definitely be the case for members of his ECC group.
The sector had already been dealing with a teacher shortage with 53 per cent of surveyed ECC members saying they're actively recruiting teachers, he said.
Of those, 69 per cent said they've received low quality applications, or no suitable candidates apply, Laube said.
"We will start to see more centres needing to downsize the number of children they cater for," he said.
However, the NZEI Te Riu Roa - the nation's largest staff union working at early childhood centres and primary and secondary schools - said it is still too early to tell how many teachers are choosing not to be jabbed.
"We are getting a high level of calls from members asking about their rights and responsibilities in regard to the mandate, both as employers and employees," a spokeswoman said.
"We don't have information about how many will not comply with the mandate, but the vast majority of our members support vaccination as the best way to keep schools and ECE centres safe for tamariki and staff."
Melanie Webber, president of the Post Primary Teachers' Association Te Wehengarua, said high schools by contrast are generally having disruptions as a result of the mandate.
"It's a tiny amount of the workforce, the vast majority have been vaccinated," she said.
Among those yet to get vaccinated, Webber said discussions were typically still ongoing, with hopes some might prefer other vaccines, such as AstraZeneca, or change their minds and return early next year.
"I'm really hopeful they will," she said.
"Because it's a horrible thing to walk away from your career, teaching is a lovely job."
Meanwhile, Auckland Primary Principals' Association head Stephen Lethbridge said feedback from principals was that this week's return by primary students to Auckland classrooms had been positive.
He said the sound of kids in the schools had made a difference and it was nice to have them back.
The number of students who had returned varied greatly depending on the community due to a number of reasons and anecdotally appeared to be anywhere between 35 to 75 per cent, he said.
There were schools across the region who had fewer children returning, or spare classrooms were able to have students back all week.
"Managing the return has meant flexibility is important."
He said as of today the APPA hadn't been told of any significant staffing issues, but it was a challenging and difficult time and a lot of work was going into making sure schools could follow and implement the vaccine mandate.