Tas DPP could have fallen asleep: doctor

A respiratory specialist has told a court Tasmania's director of public prosecutions could have fallen asleep at the wheel moments before a fatal car crash.

Dr Hugh Mestitz diagnosed Tim Ellis with sleep apnoea 18 months before a collision that killed 27-year-old Launceston woman Natalia Pearn.

Ellis's Mercedes-Benz collided with Ms Pearn's Toyota Corolla on the Midland Highway in March last year.

Prosecutors allege the Mercedes was travelling in the wrong lane.

The DPP has pleaded not guilty to a charge of causing death by negligent driving.

Dr Mestitz told the Hobart Magistrates Court he'd written a letter endorsing Ellis's driving after the sleep condition was successfully treated using an air pressure machine.

He said "sleep attacks" were not usually associated with sleep apnoea, a condition that causes sufferers to stop breathing while they are asleep.

But under cross-examination he said it remained possible Ellis had experienced the onset of sleep as he rounded a bend and crashed into the Toyota.

"I believe that is a scenario that's plausible, yes," he said.

Dr Mestitz said drowsiness in tired people was sometimes "seductive" and they could find it difficult to combat.

He said no evidence of braking or violent steering from Ellis could mean he had fallen asleep.

Crash investigator Sergeant Rodney Carrick said it was likely the two drivers had six seconds to react after sighting each other.

"I would have expected some sort of evasive action ... if the driver had perceived the hazard," he told the court.

He said a finding that the Mercedes was straddling lane markings at the time of impact was also consistent with Ellis losing consciousness.