Good catch, so just a while ago I went out to buy fried chicken and thought about what China could do if this does happen and here's what I got.
Instead of arming them similar to NATO and sending peacekeepers, China could offer to mediate the concerns by offering "security guarantees" proposals with Australia, and making the proposal public to see. Get a neutral country like Vietnam to mediate the talks between the two. After all the base is more aimed towards US than Australia.
This would make China look like a responsible world power, and promote the idea of "One cannot improve their security, at the expense of others". If Australia declines the talk, it would make them look like an irrational player, and if they went ahead with the operation, this would make them even more of a pariah than Russia, and with that condemnation, China could use the opportunity to improve their relations with the ASEAN countries. Hype up the "Australia threat" to get ASEAN on board against Australia.
Even if they didn't do the operation after the talk failed, China would win a PR battle, and Australia will be watched to see if they'll try to unilaterally change the status quo. after that, China should work behind the scenes with the government to crackdown on subversive elements and thwart any plans for color revolution.
Edit: Also US wouldn't be able to use the same strategy with China, because this would require US to bilaterally work with China, instead of unilaterally changing the status quo, also China would just tell them that they will under no circumstances the will negotiate giving away their rightful territories, then repeat the usual stuff about "One China policy" and the "Shanghai Communique"
