Applying for a business visa to China whilst living in Taiwan.

I am an Australian citizen living and working in Taiwan and have an Australian passport. If I need to visit mainland on Business do I only need the invitation letter from the supplier or do I also need a letter from my Taiwan business. I assume I don't need a letter from an Australian company as I don't work for any Australian company. Since I am living in Taiwan where do I lodge the application for the business visa?