How can there be a worst case scenario of 6.9m?

Our current population is 5.3m. How can this population shoot to 6.9m? The local population of 3.3m is unlikely to double by 2030. At less than 2.1% fertility rate, or 1.2%, the local population can hardly replace itself and likely to be not more than 4m. So where is the 2.9m coming from? It must be from immigration, the new citizens, the PRs, the E pass holders etc.

Only by import can the