The Child Support Scheme covers all children living in Australia whose parents have separated. It does not matter whether the parents were ever in a relationship and it is not relevant whether a parent spends time with their child or not. There are arrangements with certain countries for the collection and payment of Child Support, if one of the parents live in that country.
If a child turns 18 during their last year at school then that child is eligible for child support until they complete the school year. A child who is married or living in a de facto relationship is not eligible for Child Support.