Almost half of preschoolers referred to a government service because of learning or behavioural difficulties are not getting help quickly enough.
Children with a development or learning delay, a disability or behavioural difficulty that significantly affects their ability to learn can be referred to the government-funded Early Intervention Service (EIS).
Staff including advisers on deaf children, teachers, psychologists and speech-language therapists can work with children and their family until they start at school.
The Ministry of Education's target is to have at least 80 per cent of children referred to EIS receive a service within 90 days.
Only 54 per cent were within that time period in the last financial year - a drop from 63 per cent the previous year.
Officials say they are working hard to reduce wait-times and cope with record demand, and that children are not left to languish on the waiting list.
"No child is left on a wait-list without some kind of support in place, either through advice to parents, strategies for teachers or contact information to other services that may be better placed to meet a child's needs," said Katrina Casey, head of the ministry's sector enablement and support.
However, NZ Disability Support Network head Dr Garth Bennie said the situation needed to be addressed, both through funding and making sure there was not needless duplication of services.
There were 186 children on a wait-list for nine months or longer after an EIS referral in 2014/15.
"All the evidence points towards early intervention as being a very effective and sensible thing to do. [Lengthy delays] can affect lifecourse outcomes and create significant additional costs around support and services further down the track."
Green Party education spokeswoman Catherine Delahunty, who has successfully requested a parliamentary inquiry into learning disorders at schools, said EIS did good work, but was obviously struggling with demand.
"This Government is not taking special education seriously enough ... this is part of a pattern of under-investing in support."
Labour Party education spokesman Chris Hipkins said special education was a problem area and not enough money was being made available. "If kids get off on the wrong foot with their education it can take years to catch up, and some simply never do," he said.
NZ First education spokeswoman Tracey Martin said the $359 million put into the Government's "community of schools" Investing in Educational Success programme would be better directed to help front-end services such as EIS.
A spokesman for Education Minister Hekia Parata referred questions to the ministry.
Ms Casey said demand for EIS was higher in 2014/15 than it had ever been, with 14,349 under-5s provided a service, compared to 12,457 two years ago.
Some children on the waiting list would have multiple needs and would be getting support for one need while waiting for another service, she said.
The average wait time for the EIS was just over 92 days, she said - three weeks longer than for other services.
"Data we have shows the vast majority of children who are referred to receive specified support get that support within six months.
"We are working hard to reduce wait times in all areas. We are currently reviewing how we deliver special education services."
Early intervention service
• Children aged 0 to 5 years can be referred to the Early Intervention Service as soon as there is a significant concern about their learning, development, communication or behaviour.
• Percentage of eligible children receiving a service within 90 days of referral in 2014/15: Target At least 80%, Actual 54%