The Assistant Federal Police Commissioner Justine Gough, on Tuesday said an Australian childcare worker sexually abused 91 young girl for over 15 years, accusing him of documenting his "unfathomable" alleged crimes in thousands of photos and videos.

Seasoned detectives described it as one of Australia's "most horrific" child sex abuse cases, calling it "beyond the realms of anyone's imagination".

"I know this news will seem unfathomable, and I know there will be many questions," said Gough.

"There is not much solace I can give to the parents and children who have been identified," she added.

While the man was initially charged with just three offences, Gough said the gravity of his "heinous" alleged crimes emerged as police sifted further through his computer, phone and hard drive.

"This is chilling and shocking news for parents," she said.

Police believe the man filmed or took pictures of "all" his alleged crimes, and eventually catalogued more than 4,000 photos and videos of abuse.

Meanwhile, New South Wales assistant police commissioner Michael Fitzgerald said it was one of the most horrific cases he had ever seen.

"It's beyond the realms of anyone's imagination what this person did to these children," he said.

"I can only say, you try not to be shocked after a long period of time in the police, but this is a horrific case.”

Included within the 1,623 charges are 136 counts of rape, 110 counts of sexual intercourse with a child younger than 10 -- a charge used instead of rape in some Australian jurisdictions -- and 613 counts of making child pornography.

Access24 reports that Investigators had been hunting for the 45-year-old man since discovering a cache of child pornography shared on the dark web in 2014.

But their efforts had been mostly fruitless until they made an unexpected breakthrough in August last year, matching visual clues in the background of the images to a childcare centre in the city of Brisbane.