Western Australia's undiagnosed diseases program has exceeded expectations by diagnosing five out of seven children since it launched earlier this year.

After years of medical appointments and misdiagnoses, a seven-year-old Perth girl has been diagnosed with a genetic disease so rare she's the only girl in Australia - and one of less than 50 people in the world - with it.

Lily Fretwell is a success story from Western Australia's undiagnosed diseases program, which seeks to diagnose children with long-term and undiagnosed medical conditions so they can be better treated.

Lily has tricho-hepato-enteric syndrome, which affects one in one million children and adversely affects the liver, stomach, and other aspects of development.

The participants tend to have rare diseases, meaning it affects fewer than one in 2000 people.

Health Minister John Day said the program had expected a 25 per cent success rate, but five out of seven patients had been diagnosed.

"To nearly triple that figure is a remarkable achievement that is directly benefiting children and their families," he said on Monday.

UDP director Gareth Baynam said the program provided certainty and connection for patients.

"Once we know what a condition is we can remove that isolation and create a connected community for individuals, be that with other children in Australia or other children throughout the world," he told reporters.

He said a local team was at the forefront of diagnosing patients, but there was also national and international partnerships.

"We can access a global network of clinician specialists and researchers, so if we can't answer the questions we can draw on that network."

Music identity Molly Meldrum has voiced support for the program as his former personal assistant's son suffers from a rare, undiagnosed disease.

"To see the heartbreak that no one could tell them what was wrong ... it was so hard," he said.

The program is being launched in Victoria, while NSW, Queensland and South Australia have shown interest.

One child per month is chosen to take part in the program, and during their placement the UDP team do everything they can to provide a diagnosis for the child.