Business F Visa

I am an Australian currently consulting to a finance company based in Shanghai, China. I will need to be located in China for for five months this year. Can I get a 5 month F visa that will allow be to leave China on occasion to travel to Hong Kong for business meetings?

Also, I understand I require an 'invitation letter' from the Chinese company. What exactly does this invitation letter need to say? Who does it need to be stamped/signed by, any employee of the company? Or the CEO?