A fake Sydney siege Facebook post under the name of Greens Senator Sarah Hanson-Young is being investigated by the Australian Federal Police.

In a statement on Monday, Senator Hanson-Young said the post, which painted the siege gunman Man Horan Monis as a victim, was being circulated on social media networks and warned users not to share it.

Greens Senator Sarah Hanson-Young. Credit:Andrew Meares

"It's disappointing that some people have altered the statement in such a sinister way, seeking to impersonate me and politicise this tragic event," Senator Sarah Hanson-Young said.

"My thoughts remain with the victims and their families."