Canberra prosecutors are challenging the acquittal of a man accused of fathering a child with his profoundly disabled daughter after a trial hingeing on DNA evidence.
But the reference appeal can only answer questions of law, and will not change the outcome for the accused man, who cannot be named, or lead to a conviction.
The office of the Director of Public Prosecutions will argue that Chief Justice Terence Higgins erred when he acquitted the father last month in the ACT Supreme Court. The judge threw out the case at the end of the Crown case, saying they should have called evidence about whether the girl's disability might affect the otherwise strong DNA link.
But the DPP has argued the Chief Justice speculated about matters not in evidence.
The complex case came to light in 2010 when the girl's pregnancy was discovered. The girl, who was just 15 at the estimated time of conception, suffers from a moderate to severe intellectual disability, cannot speak and could not give evidence at the trial.
The disability was the legacy of a hereditary chromosomal abnormality, and also affected her brother and her unborn foetus.
Preliminary testing initially suggested the brother was a suspect.
That report was later allegedly proved faulty and a review excluded the brother but pointed to the accused, who was arrested and charged with incest.
Forensic evidence suggested there was an ''extremely strong'' likelihood the 50-year-old man fathered the foetus rather than an unrelated man picked at random.
There was no direct evidence the accused had sexual contact with his daughter, but prosecutor Margaret Jones argued the father had the opportunity, and noted there were a limited number of men known to have contact with the alleged victim.
In questioning an expert witness - albeit one without a genetics background - defence barrister Shane Gill queried whether the genetic abnormality could have affected the DNA samples.
The witness acknowledged it was ''possible'' there could be some impact on the genetic code.
Chief Justice Higgins said that while the witness would not know precisely what impact it might have, the only way to find out would be to ask a geneticist. ''The difficulty I have with it is very simply that I do not know with any degree of certainty what the effect of the genetic transposition is,'' he said.
As a result Chief Justice Higgins said he could not be satisfied of the man's guilt beyond reasonable doubt.