A 16-year-old unlicensed driver has been sentenced to eight years in juvenile detention for killing two motorists during a high speed police chase in Greenwood, in Perth's northern suburbs.

The teenager, who cannot be identified because of his age, was travelling between 155 and 170 kilometres per hour on Warwick Road in September last year when he crashed into a car carrying three members of the Forbes family.

Kevin Forbes, 66, and his wife Glenys, 60, were killed instantly. Their 25-year-old son Michael, who was driving, was seriously injured.

He was in a coma for days after the crash, and awoke to be told about the death of his parents.

The teenager pleaded guilty to two charges of manslaughter and one of dangerous driving causing grievous bodily harm.

Teen took meth, cannabis before crash

A sentencing hearing in the Children's Court was told police had started chasing the teenager's car, when its licence plate came up on their system.

Police activated their lights and sirens, but the officer who was driving slowed down after glancing at the speedometer and realising how fast he was going.

Moments later the teenager's car collided with the Forbes' vehicle.

The court heard the teenager, who admitted taking methamphetamine and cannabis before driving, was on the road because he was delivering $25 worth of cannabis to an unidentified person.

He also had a warrant out for his arrest, because five months before he had failed to turn up to court to face a charge of aggravated robbery.

Crash had an 'inevitability' to it

Prosecutor Sean Stocks said the crash happened at 7:30pm on a Saturday in a suburban area and there was nothing the Forbes could have done to avoid the collision.

"It was an unimaginable speed. At least 155 kilometres an hour in a 60 zone. There is an inevitability to what happened. Nobody was going to be able to get of his way."

Mr Stocks also said police could not be blamed for the crash, saying a message needed be to be sent to the community that "if you engage in a chase and you don't stop, you place the lives of everybody in the community at risk".

The teenager was driving at up to 170 kph when he crashed into the Forbes' car. ( ABC News )

"If somebody dies, it's not the fault of the police for pursuing you, it's your fault for not stopping."

Defence lawyer Edward Fitzpatrick said his client had an extremely dysfunctional upbringing and had been exposed to drugs, sexual abuse and other anti-social behaviour from a very young age.

Children's Court president Denis Reynolds said while he had placed substantial weight on the teenager's young age and his personal circumstances, it could not be forgotten that two people lost their lives and others were suffering the consequences of that.

The teenager will spend at least four years behind bars before being released.

With time already served, his earliest release date is in September 2020.

Couple 'lived for their family'

Mr Forbes' twin brother Keith was in court for the sentencing.

He described his brother and his sister-in-law as loving people who lived for their family.

"Our family has lost two loved ones, who will be dearly missed. Their children and their grandchildren will never see their parents again. Their parents will never be able to participate in any family anniversaries, so it makes it very sad," he said.

Mr Forbes said his nephew Michael was making steady progress but still had a long way to go.

"He doesn't remember very much about the day but he's determined, and he's undergoing various forms of treatment."