An historic World war one flag that was stolen from the RSL's Sydney headquarters 11 days ago has been returned.

The flag was found this morning in the reception area of the building, wrapped in brown paper.

The banner of the ninth battalion was carried ashore by Australian troops at Gallipoli in 1915.

It went missing while it was in temporary storage during the renovation of the RSL's headquarters, and was returned anonymously this morning.

New South Wales RSL President Don Rowe says he is relieved that the banner has been returned, undamaged.

"One of the things that helped was that there was so much media coverage that you wouldn't have been able to display it," he said.

"You couldn't have sold it. You couldn't have done anything with it. I'm extremely pleased to see that it has been returned back to its rightful place here at Anzac house