which spelling would I use in this sentence if using Australian
English?

I believe daily writing practise/practice would be of benefit to the boy.

You should use practice.

In Aus, you tend to follow UK conventions. However, even if you were following US ones, you'd still use practice. Practice is the noun in UK. Practice is the noun and verb in the US. In your sentence, it's a noun. So, practice is the right answer.

More on practice and practise.