High speed train from West Kowloon to Guangzhou

It's not available for neither 24-hour or 144 visa-free transit if traveling between HK and Guangzhou by train.
If you directly fly from Australia to Guangzhou, then to HK by ferry, you are eligible for the 144.

No, unfortunately, the train is not an eligible way how you could enter mainland China from Hong Kong using 24 or 144 hour visa free transit. You can only enter mainland China or to be precise Guangdong province by air. This is the only eligible way.

Another problem in your itinerary is the fact that you plan to return by air. So if you adapt to the entry requirement to enter Guangzhou by air, you cannot exit in the same way and remain eligible for 24 or 144 hour visa free transit because that would be a round trip and not a transit. A transit represents a journey where there has to be three countries (or eligible territories) within the itinerary. If you only have asked how to organize your visit to Guangzhou under 144 hour visa free transit, I would suggest you to fly from Hong Kong to Guangzhou and then to exit from Shenzhen by ferry to Macau. That would be a transit.

Now, I cannot see a simple solution if you have already bought your ticket from Guangzhou to Hong Kong. Maybe you could think to fly from Hong Kong to Taiwan and from Taiwan to Guangzhou. That would be also an eligible itinerary.

Please, try to think about the possibility to change your flight ticket in order to enter mainland China by air and to exit mainland China by ferry from Shekou port in Shenzhen to Macau. If you need some additional clarification, just write here and you will be assisted.