The breakthrough offers a "much clearer direction" in the search for a cure for the mysterious syndrome, which still claims one in 2,000 apparently healthy children.
Researcher Dr Jhodie Duncan, of the Melbourne-based Florey Neuroscience Institutes, studied cases of infant deaths from confirmed SIDS and other causes.
The SIDS babies were found to have lower levels of serotonin, a neurotransmitter which regulates the body's basic life-sustaining functions.
"Things like heart rate, blood pressure, sleep cycles, respiration, serotonin plays a very important role in all these things that you need to stay alive," Dr Duncan told reporters.
"Our study has proven that in infants dying of SIDS there is lower TPH2 (a related enzyme) levels and reduced serotonin production."
Earlier research had indicated serotonin production may play a role in SIDS deaths though scientists were yet to determine whether it was more, less or the same as unaffected children.
Dr Duncan said her research now pointed to a "triple risk model" for SIDS which started with a baby with serotonin deficiency.
Successful public campaigns to highlight the higher SIDS risk posed by placing a baby to sleep on its stomach had resulted in a significant decline in SIDS deaths, she said.
But a plateau in cases in more recent years indicated a baby's environment was not the only factor.
"You need a vulnerable infant ... one that has a serotonin deficit," Dr Duncan said.
"It needs to be in an environment where it becomes compromised - such as having something obscuring its breathing or a change in body temperature.
"And it needs to occur during that vulnerable period of the first six months to one year of life."
The research also provides a new insight into another of SIDS known risk factors - women who smoke during pregnancy or smoking in a home with a newborn.
Exposure to nicotine was also known to affect serotonin levels in the body, Dr Duncan said.
It is hoped the research could result in a new regime of testing, monitoring and treatment that could intervene to stop babies dying from SIDS.
"The ultimate test would be able to identify these infants and then monitor them as a high-risk infant, and intervene to alter their serotonin levels," Dr Duncan said.
"We've now got a much clearer direction in which to head ... to find a way of treating them and preventing SIDS."
The research was conducted while Dr Duncan was on a research scholarship at the Children's Hospital in Boston, and it is published in the Journal of the American Medical Association.
