Australia/Singapore/Guangzhoa/Singapore/Australia

There is no 144hr visa. There is only 144hr transit without visa, for which you must be in transit to a third country. Your proposed itinerary therefore requires a visa. Or else you must fly (for example) Singapore-Guangzhou-Malaysia and then return to Singapore.

You cannot use this itinerary to be eligible for any kind of visa free transit scheme because this is considered as a visit to China and not a transit. You can see it by yourself:

Singapore - Guangzhou - Singapore

This is obviously not a transit.

Anyway, you can be a bit creative and make an itinerary that could be definitely eligible by inserting Hong Kong in the entire itinerary. In that way, you can become eligible for 144 hours visa free transit. Like this:

Singapore - Guangzhou - Hong Kong - Singapore

You can use a train from Guangzhou to Hong Kong that takes just about 2 hours between the two cities. Of course, it would be an imperative to book the ticket for the mentioned train before your departure to Guangzhou. It is easy because you can book it on this website and you will receive an e-confirmation with your name and the passport number on it. Also, you should know that this train departs from Guangzhou East railway station and arrives in Hong Kong Hung Hom railway station. From there you can easily make a transfer to Hong Kong airport using Hong Kong metro.