A drug-affected man who murdered a woman by deliberately driving on the wrong side of a West Australian highway and crashing into her car has apologised but says he has no memory of what happened.

Shaun Southern has been found guilty of murder. Credit:Facebook

Shaun Southern steered onto the wrong side of Bussell Highway in February last year, killing 61-year-old Jennifer Pratt and seriously injuring her passenger Karen Williams.

The WA Supreme Court heard on Thursday that Southern had no memory of what happened.

"I cannot believe I did what they say I did. It's awful," he said in a report referenced in court.