the general rule is that expenses that are directly related to your trading activities are deductible, unless there are some elements of a "private nature" in the consumption of that expense resource in which case, subject to circumstance, might have to be claimed on a proportional basis.
Some types of expenses are so blatantly of a private natire that, no matter how hard you attempt to justify some "business component", will usually be disallowed by the ATO on audit.
Don't forget that we now have a regime of self-assessment, someone may trumpet that he has claimed such and such and got a refund, though technically such claim should not have been made. Beware that if that person is audited by the ATO and the claims disallowed, of course tax will have to be paid plus penalty (which is not deductible).
