A man charged over the death of a cyclist in Mandurah in 2016 may have been in an "automatic state" at the time of the accident, according to his lawyer.

Keith Michael Osman is on trial in the Perth District Court over the tragic death of Gabriele Schiller-Brett, who died after being knocked off her bicycle about 9.35am on March 29 in Halls Head.

The trial, which is expected to run for six days, started on Thursday with opening statements describing the "terrible accident" which cost Ms Schiller-Brett her life.

The court was told Mr Osman, who was 73 at the time, had been driving his Ford Fairmont on Old Coast Road when he failed to give way at a McLarty Road roundabout.

Mrs Schiller-Brett, who was wearing a helmet and light clothing, was hit by Mr Osman's vehicle and dragged for some distance before landing on a grass verge.

The Mandurah woman was critically injured and given CPR at the scene, but died shortly after from multiple, severe injuries.

Mr Osman is alleged to have slowed his vehicle "for a few seconds" following the crash, before continuing on to the Mandurah library and then heading home.

Police investigating the accident showed up at Mr Osman's Erskine home later that day, and found damage to the front of his car, but Mr Osman told officers he had no knowledge of any collision.

Also found under the vehicle was DNA matched to Mrs Schiller-Brett.

Mr Osman was subsequently charged with failing to report the crash, failing to render assistance following a crash, and dangerous driving causing death.

Defence lawyer Sam Vandongen told the jury the fact his client's vehicle had been involved in a collision was not in dispute.

Rather, Mr Vandongen raised the possibility Mr Osman had not been driving the car at all at the time of the crash, or, if he had been, had been suffering from a medical condition.

"He had no idea he'd been involved in an accident," the lawyer said.

"[Mr Osman] has no memory of being involved".

Mr Vandongen said had his client been the driver, he had been "clearly affected by something" at the time of the collision.

He said an expert witness would be called during the trial to give evidence on the possibility Mr Osman, who is a diabetic, suffered a hypoglycaemic episode at the time which resulted in an "automatic state".

"If we can prove he was not aware of the occurrence of the incident, then he's not guilty of failing to stop," Mr Vandongen said.

The trial continues.