24 hour visa

Ian, the first and the most important detail is whether you are eligible or not for 24 hour visa free transit!

To know this detail, you have to calculate your total transit time within mainland China. It is calculated from your first scheduled landing, in your case in Guangzhou, until your last scheduled departure, in your case from Harbin. If the total time you need for this transit is less than 24 hours, you will be eligible for 24 hour visa free transit and you don't need any kind of visa, but if your total transit time is longer than 24 hours for at least a minute than you will need a Chinese transit (G) visa obtained in advance.

As far as your question is concerned is, no you will not need a visa to exit the airport. You will be perfectly allowed to do that.

You will print out your flight tickets and have those printouts with you. Also, you will declare at check-in in Melbourne that you will travel to Vladivostok via China using 24 hour visa free transit. Your tickets will be shortly inspected and you will be allowed to board the plane. One more important thing that you have inform yourself in Melbourne is whether your luggage will be tagged all the way to Vladivostok.

Upon your arrival in Guangzhou, you will immediately approach 24 hour visa free transit counter where you will apply for a temporary entry permit that will be granted to you because your next flight segment will be a domestic Chinese flight. So, you will be allowed to exit in both Guangzhou and Harbin, time permitting, of course.