Remember some 1500+ Victorian healthcare workers caught Covid despite taking massive precautions.
In some circumstances this virus just gets out.
There may not be a mistake.
It could also be someone in hotel quarantine who only became infectious on Day 15 when they had returned home. (Overseas studies have shown incubation of up to i believe 17 days at the far outer edge)
NSW was always the most likely to have a break given.
1. It's the most populous state
2. It's been doing the heavy lifting in overseas returnees
Unless you stop every Australian citizen returning and stop all trade (no new cars, TVs, clothing, much food etc) you can't stop all points of risk.