Sydney - Guangdong - HongKong - Madrid ... 72 or 144hr visa-free?

Where in Guangdong? Your plan may qualify for 72 hours transit without visa (it’s not a visa, and not 144 hours) only if you travel between Hong Kong and Guangzhou by air, and if your passport qualifies.

Thanks Saul...
1. We are visiting Guangdong & Shenzhen... then, we were planning to travel to HK by train.
2. We are heading off to Spain afterwards we could take the train back to Guangdong to continue our trip - if that would help.
3. Or we could do it in reverse. Fly into HK, train it to Guangdong & Shenzhen, and fly out of HK.
By the way... thanks so much for your advice/help.
Very much appreciated.
Guy & Siani

The 72h twov just allows you to enter/exit China by aircraft.

You need a Chinese visa for this.