I will be very conservative. Lets use your number of 100,000 and assume 3 children per couple which is around a multiple of x1.5 per person
Assume they have kids at 3 children by the age of 25. Not taking into account immigration.
After 10 generations with an average of 3 children each generation the population would be around 6 million after 200yrs and 3 generations living therefore 18 million in real terms.
Australia currently has around 2.1 children per couple or 1.05 per person. It's considered zero population growth.
Assume today that 6 million are child bearing with 2.1 children per household and population growth of 1.05 per person. In 250 yrs there would be 9 million children and 3 generation living so 27 million population in real terms.
Therefore Islamic growth from 100,000 to 18 million within 250yrs and current Australian population 22 million to 27 within 250 yrs.
