24 hrs transit

Avijit, the first and the most important detail is that you are eligible for 24 hour visa free transit and you don't need any kind of visa.

You will print out your flight tickets and bring those printouts with you. Since you are traveling on China Southern and having such a long layover, you are entitled for a free accommodation. You will have to visit China Southern website and make a reservation online or do this upon arrival in Guangzhou airport. Doing it online is much better.

Also, you will declare at check-in in Dhaka that you will travel to Adelaide via Guangzhou using 24 hour visa free transit. After a short check of your flight tickets, you will be allowed to board the plane. Upon your arrival in Guangzhou, you will approach 24 hour visa free transit counter where you will apply for a temporary entry permit to be allowed to exit the airport. You will present your flight tickets and your accommodation provided by China Southern. As you already know, this temporary entry permit is issued under absolute discretion by immigration authorities, so you can really never know whether you will granted a permit or you will have to wait for your flight for Adelaide at the transit zone of the airport. Hopefully, you will be granted a temporary entry permit because your layover is really very long.