A Melbourne forensic scientist has questioned key DNA evidence that led to two migrant workers being sentenced to death for the murders of two British backpackers on an idyllic Thai island. 
Jane Taupin said documents detailing how Thai investigators matched DNA from Myanmar workers Zaw Lin and Win Zaw Htun to the victims were not provided to a Thai court, in contravention of international DNA analysis and reporting standards.
She also said that DNA matching, a complex procedure requiring meticulous care, could only be determined on the basis of statistical probability in the population and none was presented to the court.
The men have denied murdering David Miller, 24, and raping and murdering Hannah Witheridge, 23, on Koh Tao in the Gulf of Thailand on   September 14.
The Christmas Eve convictions prompted protests across Myanmar and strained ties between the neighbouring countries.
The case has also focused attention on Thailand's treatment of millions of migrant workers and damaged its tourist industry.
The men, who were working on the island at the time, were arrested after weeks of intense pressure on police from Thailand's military rulers, the media and diplomats to solve the crime.
Police said the pair confessed to the killings but both men later retracted their statements, saying they were tortured.
After a 21-day trial, three judges of the Samui Criminal Court found the accused 22-year-olds bludgeoned Miller to death with a hoe and then raped Witheridge while she lay unconscious after a late night beach party.
In a review of the DNA evidence, Ms Taupin said no documents detailing the collection, movement, handling and chain of custody of DNA samples were provided to the court which is required under United States and British codes of practice.
"The scientific records were not provided for review [for whatever reason] and thus as a scientist I could not perform a scientific review, or determine whether these records accorded with the principles of the standard," she said.
Ms Taupin, an independent consultant who has examined DNA evidence for police agencies in Australia and Britain and has received several forensic science awards, travelled to Thailand expecting to testify in the case in   July but was not called to the stand.