Babies and toddlers may be suffering toxic levels of stress because of unsafe minimum staffing in early chidhood centres, advocates say.
In some cases, there were not enough suitably qualified adults to ensure children were reassured and happy, they said.
Regulations require one staff member for every five children under the age of two, and one for every 10 children over the age of two.
Victoria University Institute for Early Childhood Studies director Carmen Dalli said the ratio for under-twos was inadequate.
She was repeatedly told of situations where children were left to cry, creating the type of stress that harmed their ability to learn and develop.
"If babies, infants, are in high stress environments for a long period of time, then that stress becomes toxic," she said.
"When they cannot have access to adults who will soothe them and help them deal with those high levels of stress then they shut down, they just basically go into withdrawal."
The government should lower the ratio for under-twos to one staff member for every three children - the same as the ratio used in England and better than the one to four that applied in Australia.
"In some research you actually find ... one to four is good enough, but one to three is ideal and we really need to move to this ideal," she said.
The owner of five early childhood centres, Maria Johnson, said the minimum staffing ratios were not safe, even for older children.
"One to five for under-twos is ridiculous and I think one to 10 is just not manageable - you just don't get safe environments for children," she said.
"Early childhood education is for care, but it's also for education, and so we need to make sure that the facilities are providing both.
"I don't think that's happening here in New Zealand anymore."
The government should require one teacher for every three children under two, and one teacher for every eight children over two, she said.
However, Evolve Education Group said the current ratios provided flexibility for its centres, which generally had more teachers than the legal minimum.
That meant a teacher could leave the room and staffing levels would still be within the legally-required ratio.
Improving staff ratios for infants and toddlers and regulating the number of children who could be grouped together were Labour Party election policies.
Education Minister Chris Hipkins said they would be part of the government's strategic plan for early childhood education.
"One of the things that the current government is committed to doing is looking at whether we've got the child to teacher ratios right, whether we've got the minimum qualification level requirements for those teachers right."
A blueprint for change should be ready by the end of this year, Mr Hipkins said.
Te Rito Maioha Early Childhood New Zealand chief executive Kathy Wolfe said it actively supported better ratios for under-two-year-olds.
"Children at that age need more focus, we'd like it one-to-three, I think that would be a good ratio," she said.
The government should also consider introducing a new ratio for children aged two to three, she said.
Early Childhood New Zealand chief executive Peter Reynolds said a one to four ratio for under-two-year-olds was a good target, and most early childhood services were already working at that level.
Any plan to lower the ratio to one staff member for every three children under the age of two needed further debate, he said.
"Lowering the ratio like that is going to increase some costs - so who's going to pay, and secondly, where are the teachers going to come from?"