Senior Sergeant Richards said police inquiries had been somewhat hindered because certain "business and agency" records which may have helped detectives with the investigation had been destroyed.
"I'd be lying if I said it hasn't had its challenges; 20 years is a long time," he said.
"We've had issues of documents and records being destroyed ... and potential witnesses who are now no longer with us."
But Senior Sergeant Richards said police were approaching the investigation with an open mind and Ms Lockyer's family and police remained hopeful she and her daughter were still alive.
"Veronica and Adell's family along with the WA Police Force are hopeful that they're alive and well, however it has been a significant period of time," he said.
"The fact they've been missing for 20 years ... that alone gives us serious concerns about their welfare."
Ms Lockyer is described as Aboriginal, 162cm tall, with a medium build, brown hair and brown eyes.
She was physically impaired on the left side of her body and often wore her hair over the left side of her face to cover her eye.
Anyone with information is asked to call Crime Stoppers on 1800 333 000 or make a report online at
www.crimestopperswa.com.au.
https://www.canberratimes.com.au/na...child-missing-since-1998-20190314-p51493.html