The collision happened yesterday when a north-bound sedan veered head-on into the path of fully loaded log truck just south of Ross.

The impact was so severe that police initially had difficulty identifying how many victims were in the car and whether they were male or female. 

Police at the scene were unable to say how the crash occurred, but said a medical condition may have caused the driver of the Ford Falcon to swerve into the wrong lane.

Deloraine division's Sergeant Phil Summers said he believed the couple were from the northern part of the state. 

The truck driver was shaken but uninjured and was taken to Campbell Town police station for interview. 

``The car has moved over onto the incorrect side of the road and impacted with a laden log truck,'' Sergeant Summers.

``The vehicle is quite a mess but we believe there are two victims in the vehicle.

``We'll certainly be following through on any possible car defect; road conditions may be a factor.''

The fatalities follow the death of a 15-year-old girl at Sidmouth last Friday after she was hit by a car on the West Tamar Highway and a truck rollover on Sunday at Police Point that killed the 42-year-old driver. 

Yesterday's crash happened about 1.30pm near the Mona Vale turn-off on a bend on a two-lane section of the highway.

The incident blocked both lanes for several hours causing heavy traffic delays. 

Heavy machinery was brought in to remove the vehicles from the highway and the road was reopened about 10.15 last night.

Northern crash investigation services will continue inquiries into the accident. 

Tasmania's road toll so far this year stands at 21.