A normally "intelligent" and "selfless" Perth primary school teacher has been sentenced to five years in jail for causing a fatal, head-on collision while she was under the influence of methylamphetamines.

Key points: Amy Asplin used ice the night before she fell asleep at the wheel, killing Dianne Dunstall

Amy Asplin used ice the night before she fell asleep at the wheel, killing Dianne Dunstall Ms Dunstall's partner was injured and told the court he had lost the will to live

Ms Dunstall's partner was injured and told the court he had lost the will to live Judge Laurie Levy said he could not understand why Asplin "selfishly" used the drug

Amy Louise Asplin, 35, had smoked and snorted the drug the night before she drove on Armadale Road in Piara Waters about 5:20pm on April 19 last year.

Her car was seen "gradually and slowly" drifting onto the wrong side of the road just before she crashed into a minibus, killing front-seat passenger Dianne Dunstall, 52, and seriously injuring her partner, Anthony Shute.

Asplin's partner, James Hepburn, who was a passenger in her car, was also seriously injured.

Asplin pleaded guilty to charges of dangerous driving occasioning death and grievous bodily harm, with the District Court hearing she accepted she must have fallen asleep at the wheel.

Asplin's tragic past outlined in court

State prosecutor Owen McLintock said an analysis of a sample of her blood found she had a level of methylamphetamine in her system that left her incapable of properly controlling her vehicle.

Asplin's lawyer, Terry Dobson, outlined his client's tragic past, which included the 2015 sudden death of her husband, who was her "childhood sweetheart", and her renal cancer diagnosis, which required her to undergo various treatments including chemotherapy.

Mr Dobson said Asplin had only used methylamphetamine once or twice, when she was in her teens or early twenties, and when she set out on her journey last year she did not believe she was still intoxicated.

He described the 35-year-old as "an intelligent person who had a good record in the community" and who was truly remorseful for what she had done.

Judge Levy said the "ripple effect" the crash would have on emergency service workers was enormous. ( ABC News )

You are 'every road user's nightmare': Judge

Judge Laurie Levy said he was "struggling to understand" how such an intelligent school teacher could "selfishly" decide to use methylamphetamine when its negative effects were so widely known in the community.

"I have pondered at length why it was you even contemplated taking the drug, let alone deciding to get in the car and drive," he said.

He also highlighted that one of the lesser-known effects of the drug was the need to fall asleep, saying there were a frightening number of meth users on the roads.

"The frightening reality is that you represent every road user's nightmare," he told Asplin.

Asplin accepted she fell asleep at the wheel and crashed into the minibus. ( ABC News )

Judge Levy said he had received numerous references which described Asplin as ordinarily a selfless character and of being of a caring and compassionate person, with a high degree of professionalism.

He also noted that Asplin had written a letter of apology to Mr Shute in which she said she truly wished that she had died rather than Ms Dunstall.

Judge Levy said Mr Shute had been left devastated by the death of his partner, and in his victim impact statement said he had lost the will to live.

Mr Shute also continued to suffer from his injuries and he said it was painful for him to walk.

Judge Levy said "the ripple effect" of what happened on the day of the crash was going to be enormous, affecting not only Mr Shute and his family, but many other people including the police and ambulance officers who attended the scene.

He sentenced Asplin to five years in jail. She will have to serve three of those before she can be released on parole.

Her driver's licence was also disqualified for five years.