Do I need a Chinese visa for these train trips? Can you please advise.

I assume that you are an Australian citizen. Visa is not required when you take train to HK. But you have to apply one when you go back to Guangzhou by train. When you are in HK, you can go to apply from the Office of the Commissioner of the Ministry of the Foreign Affairs in HK or ask the HK's biggest travel agency to handle it for you. The agency has many branches in HK. You can easily find it.