Axe murderer William Mitchell has been denied parole over the brutal killing of a mother and her three children 20 years ago, considered to be one of Western Australia's most horrific crimes.

Mitchell became eligible for parole this week after serving 20 years in prison for the 1993 murders of Karen MacKenzie, 31, Daniel, 16, seven-year-old Amara and Katrina, five.

The state's Prisoners Review Board considered Mitchell's case and sent a report to Attorney-General Michael Mischin.

After considering the report, and meeting with the victims' family, Mr Mischin knocked back Mitchell's parole on Monday.

He cited three factors including the nature and gravity of the offences committed and the degree of risk to the community if Mitchell was released.

"(Also) comments made by Justice Owen during sentencing that the offences were so serious as to defy description and that as far as he was concerned, he will never be released, at least not for many years greater than the 20 years minimum for a sentence of strict life imprisonment," Mr Mischin said.

The victims' family has been notified of the decision.

Earlier this year, a childhood friend of Amara's gathered almost 1500 petition signatures urging authorities to keep Mitchell in jail for the rest of his life.

The petition was tabled in state parliament.

Mitchell was originally sentenced to an indefinite term of prison, but that decision was overturned in 1996.

He was transferred to medium security at Bunbury Regional Prison in 2009.

Tom Percy QC, the President of the WA branch of the Australian Lawyers Alliance, said last week it would be very hard for any politician to ever release Mitchell from jail.

"Any elected MP would find it very difficult to come to that decision about someone whose odium and contempt in the public domain is so gross," told Fairfax radio.

Mr Mitchell will be reviewed again by the Board in September 2016, prior to his next statutory review date.