CANBERRA (DPA) - A 25-year-old Australian man who admitted to killing his three young daughters - twin two-year-olds and a three-year-old - and their mother and grandmother was on Friday (July 19) sentenced to life in jail.

The five were found dead inside their home in suburban Perth in September after Anthony Robert Harvey, the killer father, walked into a police station in the Pilbara region, about 1,500km north, and provided information about the incident.

Supreme Court Justice Stephen Hall said Harvey's crimes were exceptionally horrific and sentenced him to become the first person in Western Australia to be never released from jail, a provision that was introduced to the state's homicide laws in 2008.

"There is no other case that is truly comparable," Mr Hall said, adding that the women were unsuspecting, while the children were asleep.

"This is the place they should have been safest," Mr Hall said. "They should have been able to trust their father to protect them."

The court was told Harvey had written in a journal about embracing his "darkness and animal instincts" and "eliminating" his family.

"I am no psycho. I feel. I feel too much, I always have... I will regret what I do," he wrote, according to Australian news agency AAP.

Mr Hall said the journal entries were "not a mere record of dark fantasies" and added that Harvey had planned the murders for days.