A driver who was high on meth when he drove his four-wheel-drive the wrong way down a busy WA highway and killed an innocent motorist has been sentenced to life for murder.

Father-of-two Shaun Troy Southern, 46, was found guilty by a jury of killing 61-year-old Jennifer Pratt when he drove at high speed into her car on the dual-carriageway Bussell Highway, near the town of Capel in WA's South West on February 18, 2017.

Southern was also found guilty of attempting to murder Ms Pratt's passenger, Karen Williams, who was seriously injured.

He did not know either woman.

He will have to spend at least 22 years in jail before being considered for release.

His trial in June was told Southern had a $400-a-day methamphetamine habit and had used the drug for the past five years.

He had meth in his system at the time of the crash.

Footage filmed by shocked bystanders and played to the court showed Southern driving the wrong way down the highway, which has a speed limit of 110 kilometres per hour, and deliberately swerving into the paths of oncoming vehicles, before crashing head-on into Ms Pratt's car.

The state argued that Southern had the intention to kill whoever was in Ms Pratt's vehicle at the time.

Shaun Southern murdered Jennifer Pratt by deliberately driving into her SUV. ( Facebook: Shaun Southern/Jennifer Pratt )

Justice Michael Corboy said Southern's methamphetamine intoxication "might explain what was otherwise inexplicable".

He said Southern had driven on the wrong side of the road "for a considerable distance" and many other motorists had to take evasive action to avoid him.

He described Southern's actions as "appalling" and said the fact there were only two victims was "not the result of any judgement or restraint" by Southern.

Other drivers' actions praised

Justice Corboy commended other motorists who followed Southern and recorded what was happening after they saw him driving erratically, saying they had acted "maturely and responsibly."

In a statement, the families of Ms Pratt and Ms Williams "wholeheartedly" thanked those people, saying they "had put themselves at risk in order to serve their community".

"Filming the event and providing the evidence … has made the process of achieving justice so much easier than it could have otherwise been," they said.

The families of the victims have asked for privacy. ( ABC South West: Anthony Pancia )

They also thanked the communities that had helped them as well as the WA Police, prosecutors and the court victim support services.

"To our families, workplaces and friends who have kept us going, we love you and can't thank you enough," they said.

In his sentencing comments, Justice Corboy asked that the families now be allowed privacy, noting that in their victim impact statements Ms Walker and Ms Pratt's family had said constant inquiries by the media had added to their distress.

Justice Corboy acknowledged that the media played an important role in the justice system, but he noted that as a result of the inquiries, Ms Pratt's daughter had been forced to change her mobile telephone number.

"This is an intensely personal and difficult experience … they must now reconcile what has happened and they should be given the space to do that in private," he said.

"They wish to re-establish their lives without the burden of public interest in them."

Southern was also sentenced to nine years' jail for attempting to murder Ms Williams, but he will serve that term at the same time as the life sentence.

Outside the court, Detective Sergeant Stephen Cleal from the homicide squad said police were having to deal with drug-affected drivers on a daily basis.

"This investigation is a tragic example of the effects someone driving with illicit substances in their system can have on innocent members of the community," he said.

He hoped that the sentencing of Southern would bring the victims' families some closure.