On Monday morning, staff in the Melbourne newsroom were addressed by Fairfax's group director of news and business media, Sean Aylmer.

Mr Aylmer said Fairfax Media had been conducting a thorough investigation since it first heard of the complaint two weeks ago.

Mr Forbes would not be coming back into the office, he added.

"Mark felt that in this instance he had fallen short of the behaviour that The Age stands for," Mr Aylmer said.

"We acknowledge Mark's extensive service to Fairfax spanning more than 30 years, in various roles including News Director of The Age, Editor of The Sunday Age, defence and foreign affairs correspondent in Canberra, as well as Indonesia correspondent," Mr Aylmer added.

In an email to staff, Mr Forbes said he would be stepping down and leaving Fairfax Media.

"As editor-in-chief of The Age I must uphold all of the standards that we would expect in others," he wrote.

"I acknowledge and accept that I have not done so ... accordingly I believe it is untenable to remain as editor-in-chief.

"I again apologise to the person concerned, and I apologise to all of you as, ultimately, I have let you down."

It was the first official meeting with staff about the investigation.

Mr Forbes was appointed editor-in-chief in April and has been with the newspaper since 1985.

Age editor Alex Lavelle will  oversee the newsroom.