How to get Invitation letter from the competent office of the Chinese government?

I am a PhD student at the university of Melbourne and have to visit Shanghai to present my research paper in a conference. I am a Pakistani national and have valid Australian visa. I have all other documents except the invitation letter from Chinese government. I asked the conference organisers (Shanghai Jiao Tang University) but they could not provide me this letter. How can I get this letter to avail this opportunity?