We'll do what we should do with any other country when one of our citizens is on trial for any crime. Observe the trial, comment diplomatically if we feel it is unjust, and generally respect that nations laws, just as we would hope they would respect ours if the situation was reversed.
Unless it's someone of high government importance China would not give a * if Australia executed one of their citizens for drug trafficking. That is obviously a hypothetical considering Australia don't.