An elderly driver was in "an automatic state" and not doing "a willed act" when he hit and killed a cyclist in Halls Head, a Perth court has been told.

Keith Michael Osman, 74, is on trial in Western Australia's District Court accused of dangerous driving causing the death of 69-year-old Gabriele Schiller-Brett on the morning of March 29, 2016.

It is alleged Mr Osman failed to give way to Ms Schiller-Brett as she rode through a roundabout, crashing into her and dragging her and her bicycle underneath his car.

He is also charged with failing to stop and render assistance, and failing to report the crash to police.

State Prosecutor Les Hobson said after hitting Ms Schiller-Brett, Mr Osman continued his journey, driving to Mandurah where CCTV cameras later recorded him spending time at the library.

He then returned home, where police found him and examined his car, finding evidence that it had been involved in the crash.

The court heard Mr Osman told the police in a video record of interview, he had "no knowledge of any collision."

Driver's mind 'not connected to body'

Defence counsel Sam Vandongen, SC, said the interview was "a very important piece of evidence" because Mr Osman also told the officers that he had driven "a totally different" route to the library, and he had "no memory whatsoever" of being involved in a crash.

"What we say he told the police is the God's honest truth, in particular that he had no idea he had been involved in an accident," Mr Vandongen said.

Mr Vandongen said the defence would be arguing Mr Osman was "clearly affected by some medical condition he had," which could have been sleep apnoea or type 2 diabetes.

He said Mr Osman had been a diabetic for many years and people who suffer that condition sometimes had a hypoglycaemic episode when their blood sugar drops to dangerously low levels.

"It affects a person's ability to control their actions. It can result in an automatic state where their mind is not connected to their body," Mr Vandongen said.

"This is what we say has happened to Mr Osman on that day."

Mr Vandongen also highlighted his client's actions after the crash, which included spending about an hour-and-a-half at the library — located across the road from the police station — and then going to a fast food outlet, before returning home and parking his car in the garage.

"This odd behaviour, we say, is not consistent with someone who has done a willed act, who consciously drove through a roundabout … and ran over a lady on a bike."

Mr Vandongen also said while it was admitted that Mr Osman's car was involved in the crash, no one had identified him as being in the driver's seat.

The trial is expected to run for one week.