This is based on a false premise, that the Govt will ask developers , and that they would care if it is difficult/infeasible/impossible to actually complete. In reality, they issue a notice to the company, give them a timeframe, and expect it to be done. They don’t care about the intricacies of git.
But the problem remains the same: how do you keep it a secret? How does the backdoor not get leaked immediately to the press, to the customers of the software, etc? Sure, this strategy may work in China but Australia is a Western nation where freedom is taken seriously. Edit later: by 'freedom taken seriously I mean by the people, not by the government.'
Oh, you sweet summer child.