"Widespread suspicion" has fallen on a key figure in the government body involved in an investigation into the Australian Workers Union, who was due to take a job in Employment Minister Michaelia Cash's office.

Senator Cash's media adviser, David De Garis, resigned last week after admitting he told some reporters that raids by the Registered Organisations Commission on the AWU were about to happen.

Fairfax media reports Mark Lee works for the Fair Work Ombudsman and also does work for the commission which ordered police raids over donations made by the AWU to activist group GetUp!

Mr De Garis has not revealed where he got the information and Australian Federal Police are investigating the source of the leak but it has emerged Mr Lee, who was due to replace him in Senator Cash's office, allegedly knew about the raids hours before they took place.

Fair Work Ombudsman Natalie James denied in a Senate estimates hearing last week that Mr Lee was the source of the leak but ACTU secretary Sally McManus says "widespread suspicion" remains about Mr Lee's involvement.

"Can the FWO explain how Mr Lee avoided the conflict of interest between his current employer and his prospective employer, Minister Cash, when he had access to highly politically sensitive material last week?" Ms McManus wrote in the letter, Fairfax reports.

"If a widespread and understandable suspicion exists that Mr Lee was colluding with Senator Cash's office, why would the FWO consider it appropriate to retain Mr Lee as a media adviser?"

An FWO spokesperson says Mr Lee will continue his job as media director at the ombudsman's office.

Opposition Leader Bill Shorten was head of the AWU and also on the board of GetUp! when the union donated $100,000 to the activist group, with the investigation looking at whether the union followed proper procedures when making the donation.