I heard that when China block Australian coal, their coal were imported into China via a third country making the price even more expensive for China.
I don't know!
That would certainly happen during the initial period of sanction, there are contractual obligations, the steel maker need the coal immediately for their furnace and has no way to find new suppliers, etc.
Sanction is always a double-blade sword, the real purpose is for the long run. China has signed a long term agreement with Mongolia to replace the Australian coal last year.
In the long run, China's sanction against Australia is not about hurting Australia more, it is disengaging an unreliable supplier. Of course, if Australia's market outside of China is not able to replace China's consumption, Australia will suffer. If production outside of Australia can not satisfy China's need, China will suffer. The true outcome will take a long time to know, and both countries have some time to remedy the impact, we shall wait and see.