I just read in Wikipedia:

"Islam in Australia is a minority religious group. According to the 2011 census, 1,457,300 people, or 2.2%[1] of the total Australian population were Muslims. This made Islam the fourth largest religious grouping, after all forms of Christianity(64%), no religion (22.9%) and Buddhism (2.5%). Demographers attribute Muslim community growth trends during the most recent census period to relatively high birth rates, and recent immigration patterns.[2][3]"

So, almost a quarter of the population has no religion or are agnostic or atheist. If the Christians and the agnostics/atheists genuinely talk to each other and actively listen also rather than just hurling their own arguments at the other, then, I believe, they have no choice but to become Muslims.