if you do the 3nights one week 4 the next then you're doign 50/50 shared care and based on your incomes, yes, you'd have to pay him child support

if you did 2nights one week, 3 the next, your care percentages would be different (64/36). the child support agency would take out a set amount for living expenses for each of you, pool the rest, work out 64% of that (for you) - if you earn more than that amount of what is in the "pool" you may have to pay him some child support. if you earn less than 64% of what remains in the pool, he may have to pay you.

it is a complicated scenario - it is based on taxable income (NOT take home) so him paying HECS and getting less take home is irrelevant. the fact that he had 6 months off to care for your children might impact things though, as his income LAST financial year will be significantly reduced, and as a general rule, they base assessments on last financial year....

another that agrees with punching in the numbers on the child support site (or you can do it on the c'link/fao estimators and run the fao/child support stuff together