Australian visiting Hong Kong and China

Hi my name is Shiow and I am an Australian passport holder, currently residing in Melbourne Australia.

I am visiting Hong Kong for 9 days (I don't need a visa for that). However, we have decided to take a train/shuttle to go Guangzhou instead for this trip as we have never been to Guangzhou.

I understand that I do need an entry permit for China and there is an express/rush application service. Can you please let me know what do you need for this? Also if I don't have a return ticket to China, will my return ticket to Hong Kong be sufficient? We plan to buy the return train/shuttle ticket when we get to Hong Kong.

Thank you and please let me know.