Seems a pretty over-simplified way to judge who should get the tax relief. The original point here - that non-custodial parents should probably be given some kind of tax relief too - doesn't seem to me to automatically break down fairly and squarely on one factor such as how much custodial time they have.

With this simple system you describe, a parent who has no custody, but only has visiting access arrangements (or no access at all) would get no tax relief , even though by paying the potentially very large amounts of child support, they are possibly spending more of their earnings on raising the kid than the custodial parent is. There are cases where a person is effectively fully supporting an ex-spouse as well as the kid(s), but they might not then get any of this tax relief at all.

It all seems to need a serious rethink to me. Perhaps with the new Government on board in Australia and (so far at least) seemingly not afraid to shake things up a bit, such a rethink may actually happen. Somehow some way of determining the actual cost of raising the kid (or supporting the non-kid dependent as you mention) - then combine that with the amount of child support paid by non-custodial parent, and the contribution to it made by the custodial parent, to determine who gets what in terms of tax relief? Probably wishful thinking - whatever system one devises, people will manipulate it to their benefit (and thereby to the other persons detriment). So in any such system such manipulations should be a serious criminal offence (basically, equivalent to tax evasion).