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.
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.
..while non-religious make up a fair portion of Aussies, most still identify as Christians.
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.
Age - conservative retirement-age voters are far more likely to vote than the 18-30 demo.
Networking - churches can, and do, bully-pulpit entire parishes.
So policy is far further to the right than the population median.
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.
That's not been my experience.
Age - conservative retirement-age voters are far more likely to vote than the 18-30 demo.
Australia has mandatory voting. We have something like 98% voter turn out. You don't know what you are talking about.
Networking - churches can, and do, bully-pulpit entire parishes.
Facts not in evidence.