A British backpacker has been acquitted at a retrial of the rape of an American woman in Australia after DNA of a second man was found.

Scott Harry Richardson spent nearly two years in prison after being convicted of the alleged attack at a house party in 2015.

The 27-year-old had his conviction quashed last year after an appeal court heard new DNA evidence had come to light.

He denied the allegations but was found guilty on December 13, 2016, and was ordered to serve at least two years and three months in prison.

A jury at Sydney's Downing Centre court on Thursday returned verdicts of not guilty on two counts of sexual intercourse without consent following the retrial.

Scott Harry Richardson (pictured) has been acquitted of sexually assaulting a 23-year-old American woman at a house party in Redfern in 2015

Mr Richardson, from Bristol, had been accused of attacking a 23-year-old woman as she slept at a property in the Redfern suburb of Sydney on December 12, 2015.

After spending more than 21 months in jail, his conviction was overturned by appeal judges at the New South Wales Supreme Court in July last year.

Judges heard that DNA tests using a more advanced method than those used during his trial found profiles of two men, rather than one.

The 'Y-filler Plus' test, which was not available at the time of the trial, is said to be a more effective way of analysing complicated evidence and sexual assault samples that contain small amounts of male DNA in large quantities of female DNA.

During the original two-week trial, the court heard the alleged attack occurred while the woman was sleeping under a blanket on a couch in the early hours of the morning, following an 'impromptu' party at Mr Richardson's apartment.

Mr Richardson met the 23-year-old woman at a house party in Redfern in December 2015

The woman claimed she woke up to find her assailant on top of her, and when she told him to stop, she said he replied with 'This is what you want, isn't it? This is what you want?'.

At the time, Mr Richardson told the court he did not rape the woman, but rather it was a case of mistaken identity.

He claimed he jumped onto her thinking she was his friend. But after realising his mistake, he got off and said 'sorry' before leaving the room.

A crucial piece of evidence relied upon by the prosecution was a sample collected from the woman's vagina, which was found to include Mr Richardson's DNA as a result of 'Y-filler' testing.

Mr Richardson's lawyers argued the DNA was due to 'secondary transfer', which suggests DNA can be transferred as a result of simple skin to skin contact, such as a handshake. But the New South Wales district court rejected this claim.

The 'Y-filler Plus' test, which was carried out by Forensic Science SA, maps 27 markers on the Y chromosomes unlike the 'Y-filler' test used by Forensic Analytical Science Services (NSW), which only maps 17 markers

As a result of the findings, Mr Richardson was found guilty of two counts of sexual intercourse without consent and was serving a minimum two year and three month jail sentence.

The Court of Criminal Appeal quashed his conviction after hearing that the DNA sample has undergone more rigorous 'Y-filler Plus' testing, which revealed there were two male DNA profiles compared to the original test that showed one.

Unlike the 'Y-filler' test used by Forensic Analytical Science Services in New South Wales, which only maps 17 markers, the superior 'Y-filler Plus' test, which was carried out by Forensic Science SA, maps 27 markers on the Y chromosomes.