Joyce Clarke, 29, was allegedly armed with a knife when a constable shot her on September 17 on a residential street in Karloo, Geraldton.

The mother-of-one had mental health problems and had only recently been released from prison.

On Thursday, police said an officer had been arrested as a suspect, then advised about three hours later he had been charged.

"Members of Ms Clarke's family have been advised of this morning's developments, as have WA Police Force staff," a spokesman said.

WA Police Union president Harry Arnott said the officer was being supported.

"We stand by the police officer 100 per cent and will do throughout this ordeal," Mr Arnott said.

"Our focus is on the member's welfare and ensuring his rights are protected."

Family and supporters have demanded to know why a Taser was not used on Ms Clarke instead of a gun, and rallies were held last year calling for justice.

National Suicide Prevention and Trauma Recovery Project director Megan Krakouer told AAP she had been in contact with Ms Clarke's adoptive mother Anne Jones, who took care of her from the age of five months.

"She has conducted herself with so much dignity," Ms Krakouer said.

"There's a sense of relief. She is seeking that closure, she is seeking that justice.

"We're glad that steps have been taken to bring about justice for Joyce and we'll be watching with great interest."

NSPTRP national coordinator Gerry Georgatos said the group had been supporting 31 family members since Ms Joyce's "harrowed, tragic death".

"If an officer is charged for a police custodial death, it will be only the second time in WA history," he told AAP.

Mr Georgatos said five officers were charged over the 1983 death of 16-year-old Aboriginal boy John Pat in Roebourne, but were acquitted.

The royal commission into Aboriginal deaths in custody was held a few years later.

WA Police Commissioner Chris Dawson said at the time of Ms Joyce's death there were more than 20 witnesses, and vowed there would not be any "cover up or skimming over facts".

Police have fast-tracked the roll-out of body-worn cameras for officers in the area.

Australian Associated Press