Wrinkly's point-of-view:
It seemed to be (a Christian country) when I was growing up. I can remember once a week at school (a state school) we had RE - religious education. We were divided up into 'religions' and off we went to different classrooms for half an hour. The 'religions' on offer were C o E, Catholic, Presbyterian, Methodist, etc. I don't remember any kids being left on the bench un-catered for. It was understood that we WERE Christians, but of different flavours.
So that was a long time ago, of course, but in my mind, Australia traditionally WAS a Christian country, but we're now being out-numbered. To younger folk whose world only started in much more recent times, I can understand the different perception of how Australia is.
We're getting down to semantics here because it's kind of two different things.
Saudi Arabia is a Muslim country, by law that is the religion and many laws and practices are based on the contents of the Koran.
Australia is a secular country, the church is separated from the judiciary and the parliament and the executive. We are free to worship as we please and religious practice does not supercede the law. Of course, many of our laws are based on the ten commandments (steal, murder, etc.) but many commandments are not law (adultery, idols, etc.) and many laws we have are devoid of religious values. So it's not 100% black and white but we are not a Christian or religious country given the separation of powers and religious freedoms enforced by law.
What you are getting at is the prevailing culture. When I went to school we also had scripture classes based on denomination (****, I even went to Sunday school!) but you could opt out if you wanted and they were 40 mins a week. Yes you see many churches and Christian schools, in the majority as well, but you also see Jewish schools and Synagogs, Muslim schools and Mosques, Ba'hai schools and their temples, etc. etc. Christianity previals in terms of statistics but that's all and it is also reducing rapidly due to both the reduction in religiosity of society in general as well as the increase in other religions as well as 'alternative' religions (paganism, spaghetti monster, satanism, beerism, Zuckerburgism, etc. etc.).
I just asked a barman if he had any IPAs as we had a long almost-Christmas lunch. He said "we've got something that's almost an IPA" and handed me a pale ale.
Prick should be crucified.