One entry flight Chengdu then HK can back to Guangzhou flight Sydney?

So UK - Chengdu - HK - GZ - Sydney? Your stays in Chengdu and GZ are eligible for the 240h visa free transit so you don't need a visa for this. Just prepare the ticket to leave for HK or Sydney within 10 days before landing.

Also, for the second trip HK - Guangzhou - Sydney, you cannot travel by train from HK to Guangzhou, yo need to fly or take a ferry to Guanzhou Nansha port.