Australian officials currently estimate that about one in 160 children are diagnosed with autism, but findings from two new studies suggest it is much more common.
It is not clear whether autism itself is on the rise, or whether better diagnosis is inflating the figures.
Researchers from Melbourne's La Trobe University studied 20,000 children as they grew from infants to toddlers.
They trained baby health nurses to pick up early signs of autism.
Dr Cheryl Dissanayake is one of the lead researchers.