A Bunbury man who allegedly drove his car onto the wrong side of the road and smashed into another motorist, killing a woman and injuring another, remains under police guard in hospital.

Shaun Southern, 45, allegedly forced dozens of motorists off the road in a 20km horror run on the Bussell Highway near Capel, south of Perth, on Saturday, during which he allegedly repeatedly crossed onto the path of oncoming traffic.

Shaun Southern. (Supplied)

Jenni Pratt, 62, a mother-of-two and school teacher, was killed when the Toyota Landcruiser slammed head-on into her Toyota Rav4.

The impact propelled Mr Southern through his windscreen, who landed on the road before his vehicle caught fire.

Jenni Pratt, 62, was killed when a Toyota Landcruiser smashed head-on into her vehicle. (Facebook)

9NEWS understands witnesses reported seeing Mr Southern camping on Wonnerup beach before abruptly speeding off, leaving his wallet and phone behind.

Ms Pratt's family yesterday paid tribute to her “brightness and beautiful smile”.

“The way she listened and cared, meant that so many people loved and relied on her,” the family wrote on Facebook.

“We would like to acknowledge and thank all the people at the scene who risked their lives to try and help.”

Passenger and friend Karen Williams, 54, remains in hospital in a stable condition after being aided from the wreckage.

Her daughter Paris also thanked the witnesses, telling them their actions “meant the world”.

The impact was filmed by passer-by Dan Mateljan as he travelled with friends, who were the first on the scene.

The video shows the Landcruiser collide with the Rav4, sending huge amounts of debris into the air.

Mr Mateljan and his friends parked their vehicle and ran to the site of the crash to offer help.

Passers-by were then filmed towing the Rav4 away from the Landcruiser as a fire took hold of the vehicle.

Mr Mateljan told the TODAY show he was still in “disbelief”.

Witness Jason Johansson yesterday told 9NEWS he leapt to help Ms Williams, trying to calm her down.

“I tried to tell her everything would be okay and that emergency services would be on their way,” he said.

Mr Johansson, 50, said he had to swerve several times to avoid the driver.

Another witness, Tom Russell, said the first person he saw was the Landcruiser’s driver “laying on the floor in the middle of the road”.