Australian christians don't really advertise the fact. You won't see an Australian politician saying "God Bless 'X'" at the end of a speech (as appears to be the usual case with US politicians) unless they're speaking to an overtly christian audience. It's fine to be christian in Australia, but proselytising is very frowned upon, and christians who often publicly state their faith are generally seen as 'wowsers' (fun-killers).

Also, MPs are generally voted in by party affiliation, so people vote more for the party rather than the individual person. Anyway, I'm not sure how it's happened, but the power-brokers of both major parties have a greater proportion of staunch christians than the wider population. So, when a party wants to do X or Y, often a bone will need to be tossed to one of these power-brokers to get the numbers.