A wild guess: The most influential backers are Christians. They have the money to lend to the party so it can build effective marketing and political campaign to go after the rest of the voters.
Given that the majority of Australians are Christian it would indeed make sense that the most influential backers are Christians too. Probably been like that for a long time?
Still there might be something in their campaigns that resonates with the views of the majority. One wouldn't imagine them consistently winning fair elections in predominantly Hindu or Muslim country, no matter the campaign spending.
The majority of Australians are secular. Take me: I'm from a family that's Church-of-England (C of E). I've been an atheist since at least 10 years old (first strong memory of it), yet the first time I filled out a census form, I ticked C of E because that was my family's religion. These days the religion question on the census form is the only contentious one on it, because of things just like this. Lots of people associate with a given religion but don't really practice it - churches in Australia are fighting falling attendance and graying congregations.
If you scroll down a single comment, you'll see one from me pointing out at least one issue with the religion question on the census.
Your own link shows another - the option for 'no religion' has historically been obfuscated or separated. Once upon a time, it didn't even exist. The whole point of your linked article is about how that question can be played with in order to fudge the results.
Yes, but not particularly harshly. The Electoral Commission's target is 95% of eligible voters enrolled, and it's currently at 93.5%. You are supposed to enrol at some time between your 18th birthday and the next election (which roll around every 3 years or so), so some of that non-enrolled percentage are folks who will do so shortly. A small amount of people are eligible to vote, but not capable, such as people with severe intellectual handicaps. In practice, most folks are enrolled.
The penalty isn't particularly harsh ($20 first time missed a vote, $50 afterwards). I have a 40-year-old friend that's not enrolled, and as far as I'm aware, she has never been fined for it.
