Failure to correctly diagnose the special needs of children could lead them to self harm, it has been claimed.
One expert warned parents also fear their children are suffering mental health problems because autism spectrum disorder is not being recognised.
The Welsh Government has invested £13m in an autism service but it does not offer help to those yet to be assessed.
It said parents who have concerns about their children should contact their GPs.
Rachel Church is a tutor specialising in autism at Swansea University Medical School and previously ran an advocacy service for 10 years.
She said while there can be waits of between 16 to 22 weeks for a diagnosis of autism or Asperger’s Syndrome within the Child and Adolescent Mental Health Services (Camhs), parents may have raised concerns years before they have even referred.
“The absolute worst impact that it has on the child when they are not diagnosed and left unsupported is it leads to what parents describe as mental breakdown, but school might describe as maladaptive or challenging behaving, so they’re being categorised in school as naughty, or disruptive, or non complaint,” she said.
“There can be very long waits for Camhs, and in that time the child is still disintegrating, you see issues of self harm, breakdown, school refusal, and that’s if they get referred to Camhs at all.
“More commonly when you see aggressive or non compliant behaviour, it comes to school exclusion before it comes to Camhs.
“The child feels trapped, unsupported, anxious, and the only way they have to cope with this is outbursts.
Read more at: http://www.bbc.com/news/uk-wales-40534994