A man who drove at high speed the wrong way down a WA highway and crashed his 4WD into another vehicle, killing the female driver, had a meth habit costing $400 a day, a Perth court has been told.
Shaun Southern is on trial for murder over the crash, which happened on the Bussell Highway near Capel, in WA's south-west in February last year.
Jennifer Pratt, 61, who was driving from Bunbury in the other vehicle, was killed and her friend Karen Williams was seriously injured.
Prosecutor Justin Whalley said Mr Southern, now 46, had admitted to smoking meth from the age of 40, and his habit cost him up to $400 a day.
Forensic psychiatrist Victoria Pascu said Mr Southern tried to "self-medicate" by using methylamphetamine regularly.
The court was earlier told Mr Southern had recorded a meth reading of 0.56 milligrams per litre several hours after the crash.
Southern had 'altered mental state'
Dr Pascu said Mr Southern "did not have a documented history of long-standing mental illness", although he may have experienced depressive symptoms and stress associated with his chronic medical issues.
She said Mr Southern had suffered post-traumatic amnesia and had no memory of the crash.
Witnesses who saw the then-45-year-old soon before the incident had described his behaviour as "odd and unusual", "erratic" and "manic".
Dr Pascu told the court that this behaviour could be attributed to methylamphetamine intoxication and evidence of an "altered mental state".
But under questioning by prosecutor Whalley, Dr Pascu conceded there was no evidence of psychosis during the incident itself.
She said even acute intoxication would not have stopped Mr Southern engaging in "planned action".
The trial is continuing.
