Fly to Dhaka via Gonzghou- Sri lankan passport

You have to go to the chinese embassy and apply for a transit visa.

Beside the alternative mentioned by Thomas, you can also rely on 24 hour visa free transit option. Your itinerary is eligible for such an option and you can use it without the need to obtain a Chinese visa at all.

You will print out your flight tickets and have those printouts with you. Also, you will declare at check-in in Sydney that you will travel to Dhaka via Guangzhou using 24 hour visa free transit. After a short inspection of your tickets, you will be allowed to board the plane.

Upon your arrival in Guangzhou, you will clear all the formalities like immigration and security and after that you will proceed to your gate for a flight to Dhaka. Please, have in mind that you will be inspected for a visa that Sri Lankan citizens need to travel to Bangladesh.