I don't believe Australia or NZ has had anyone leave quarantine after 2 weeks and then test positive. The only exception might be where there was transmission inside a facility.
Actually the famous case that escaped in Adelaide quarantine was exactly that.
Guy tested negative before the flight, and on days 2, 7 and 14 of quarantine. Then drives around and spreads it. They assume he caught it on his last or second last day in quarantine.
Which is why I had to have tests 72 hours before my flight, then on days 2, 7 & 14 of quarantine, then yet another test on day 16
