A man behind bars for at least 25 years is again challenging his conviction for drowning his fiancee in the bathtub of their Adelaide home two decades ago.

Henry James Keogh has lost a number of appeals against his conviction for murdering Anna-Jane Cheney in March 1994.

But under new South Australian legislation, he has begun a landmark challenge arguing there is fresh and compelling evidence in the case.

His lawyers contend that fresh evidence, including the possibility his fiancee may have suffered an extreme allergic reaction, undermined his conviction.

READ MORE: Henry Keogh’s last, best hope

Prosecutors claimed Keogh drowned Cheney by grabbing her legs and lifting them up over her head.

But earlier this year, when arguing for permission for a new appeal, Keogh’s lawyer, Marie Shaw QC, referred to an alleged grip mark on Cheney’s leg.

Shaw said there was now evidence from a tissue sample that showed the bruise was at least three to four days old at the time of Cheney’s death.

She said that at the trial the crown had “pitched” the submission that the jury could exclude accidental drowning, referring to the grip mark.

Shaw also said there was a new report suggesting Cheney had been prescribed and used the drug Hismanal some time before her death.

Associate Professor Anthony Thomas has suggested this drug could have caused an allergic reaction that raised the possibility of anaphylactic shock being the cause of death.

The Court of Criminal Appeal, before three judges, begins today (Monday).