Here's how you can tell it was china. When it was leaked that it was China by Scomo, they didn't immediately hotly deny it, threaten Australia, etc.. all the stuff they did for Covid. They went, "It wasn't us, it could be any one" and went oddly quiet on threatening any further retaliation on Australia.
Here's the reason why, it _was_ China, and China doesn't know exactly how much Australia knows about what they've been doing. If China flatly denies it and we have evidence to prove it that we can release, not only does it put China in a bad place diplomatically (and potentially scuttle their anti-COVID crusade in the UN), but it also reveals their tactics to every other country in the world with whatever Australia released.
If it wasn't China, they'd be daring us to prove it left right and centre instead.