Australians who've been treated for clinical depression are needed for a large study designed to detect genetic factors that contribute to the mental illness.

Scientists are seeking 20,000 Australians who have been treated for clinical depression to take part in the world's largest genetic investigation into the devastating illness.

The Australian Genetics of Depression Study is designed to detect genetic factors that contribute to clinical depression, in order to develop better treatments and ultimately find a cure.

Currently people with clinical depression are often blindly prescribed medication in the hope it will work and have few side-effects.

But their effectiveness is often not known until weeks later and in many cases treatment is successful for some people and not for others for unknown reasons.

Understanding the "genetic architecture" of depression will help to solve this problematic situation, says co-investigator and mental health campaigner Professor Prof Ian Hickie, AM from the Brain and Mind Centre at the University of Sydney.

"In psychiatry we have really suffered because we've been stuck with clinical categories that don't predict very well the response to treatment," Prof Hickie told AAP.

"Bipolar depression is a great example of that because within that group you have people who do really well with anti-depressants and some people who do hopelessly and only have severe side-effects."

Professor Nick Martin, Head of the Genetic Epidemiology group at the QIMR Berghofer Medical Research Institute, is co-leader of the study and says they're seeking volunteers aged 18 and older who are currently being treated or have been treated in the past for clinical depression.

Volunteers will need to complete a 15-minute online survey and donate a saliva sample that will be screened for hundreds of DNA variants through a process known as 'genome-wide association scans' (GWAS).

GWAS will allow researchers to look for genetic similarities and differences, which will help find more personalised treatments, says Prof Martin.

"All the known medications that we've got are working on fairly limited knowledge of the biochemistry behind susceptibility to depression, so what GWAS does is lay bare all of the different pathways that are involved."

But in order for the study to be successful they need a huge number of people to volunteer.

"Study volunteers will be making a genuine contribution to better understanding and helping us to solve this devastating illness," said Prof Hickie.

Clinical depression will affect one in seven Australians in their lifetime.

It is a severe pathophysiological syndrome that changes the body's whole physiology.

"It isn't a simple reaction to an unfortunate life event or difficult circumstance, so it's not transient period of low mood," said Prof Hickie.

"The amount of disability and impairment and loss of employment and impact on family is very high right now, so that's really what we want to change through more effective treatments," he said.

To volunteer for the Australian Genetics of Depression Study or to learn more, head to: Web:www.geneticsofdepression.org.au

Email: gendep@qimrberghofer.edu.au