Two NSW men, including a former childcare worker, will likely spend decades behind bars after being sentenced for hundreds of child abuse offences described by police as "some of the most evil crimes imaginable".

The two men, who cannot be named for legal reasons, were arrested in June 2020 as part of a major operation led by Australian Federal Police (AFP).

They pleaded guilty in 2022 to 354 child abuse offences between them, which involved 30 victims.

The older man, aged 30, was convicted of 248 offences and was sentenced in the NSW District Court to a maximum 37 years in prison, with a non-parole period of 26 years.

His younger co-offender, aged 25, was convicted of 106 offences and was sentenced to a maximum of 26 years with a non-parole period of 16 years and nine months.

Each man's offending included more than 30 counts of sexual intercourse with a child under 10 years of age.

The two were among 25 people across the country charged as part of the AFP operation, which began following a tip off from the National Centre for Missing and Exploited Children in the US.

'Truly some of the worst offending we have ever seen'

AFP Commander Kate Ferry said the investigation, known as Operation Arkstone, started as a result of "one small piece of information".

"What the AFP and its domestic and international law enforcement partners uncovered in the weeks and months that followed was truly some of the worst offending we have ever seen," she said.

"The criminal behaviour of these two men is perhaps the most disturbing representation of what child sex offenders are capable of, being the systemic sexual abuse of children over many years, across geographical locations and by people who have been entrusted with so much responsibility."

Commander Ferry said some of those arrested were meant to keep children safe.

"Instead, they used their position to commit some of the most evil crimes imaginable."

The AFP said the "digital trail" uncovered during the investigation identified 56 child victims who were removed from harm.