More than a thousand Australian families have been contacted by NSW Police and the Australian Federal Police (AFP) over an alleged sex offender.

The man worked with children in the northern suburb of Sydney and the CBD, and is at the centre of an ongoing investigation.

He was charged by the AFP in October last year with multiple offences and remains in custody.

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“Investigators believe the alleged behaviour of the man involves a very small number of children, however, the parents and carers of any child who may have had contact with the man has received a letter,” the AFP said.

“Parents and carers of children who may have had contact with the man have been informed of the investigation and provided important information regarding the health and support services available to them and their children.”

More than 1200 families have received a letter, as reported by the Sydney Morning Herald.

The letter gave parents and carers instructions on how to contact a local hotline if parents or carers have concerns about their child.

“Investigators ask those who receive the letter to read it carefully and contact the local hotline, as per the instructions, if they believe they require further assistance,” the AFP said.

The man cannot be identified due to non-publication orders.