Police said the 18-year-old was on his way to work about 2am yesterday when his Subaru Impreza hit at least two cows on the Bass Hwy, west of the Backline Rd junction.

The vehicle - which was extensively damaged in the crash - came to rest down an embankment.

The driver, who received serious head, neck and chest injuries, was flown to the Royal Hobart Hospital, where he remained in a critical condition last night.

Police did not release the name of the injured man, who is believed to live with his parents near Forest.

Sgt Lee-Anne Walters, of Smithton police, said yesterday about 30 black cows had wandered onto the road before the crash.

Another two motorists had frightening encounters with the livestock, all three incidents occurring in the space of about 15-20 minutes.

In the second crash, Sgt Walters said a steer came to rest on the back of a Holden flat tray after it was struck.

Extensive damage was caused to the passenger side of the vehicle, but the driver was lucky to escape injury. A fully laden B-double log truck was the third vehicle to strike cattle.

This time only minor damage was caused to the vehicle and the driver was not injured.

Sgt Walters said black cows on a pitch-black night, with strong winds and rain, were a dangerous mix.

"You're looking at half a tonne of cow and a highway speed limit of 100kmh - you're going to do some damage," she said.

There was no indication that any of the drivers were to blame for the crashes, Sgt Walters said.

"There's nothing to suggest they weren't driving to conditions," she said.

Five of the cows had to be destroyed while the others were successfully rounded up.

Livestock owners are responsible for ensuring their animals do not journey onto roads.

Sgt Walters said the owners in this case had been spoken to by police.

Inquiries were continuing, but she said the adverse weather conditions may have spooked the animals.

Sgt Walters said Smithton police were also grateful for the assistance of SES volunteers and members of the public in helping to round up the cattle.