Child support is payable until the child turns 18, or until the child is married or enters into a de facto relationship, but your partner will need to contact the child support agency to inform them of the change of living arrangements. For the record, a de facto relationship is only considered a de facto relationship if certain thresholds are met, such as being in a relationship for two years, sharing a child together or having the relationship formally registered as a de facto relationship in the state of Queensland. A call to CSA should clarify his obligations easily enough.