When the US led forces bombed IS in Iraq it was a formal invitation of the Iraq government in a bid to stop IS. However the situation is Syria poses a different scenario. The Syrian government has not endorsed the bombing of IS, there is no UNSC mandate and the US/Aus is not under direct threat of attack.
The bombing appears to be against the International Law of War. Given the above under what legal justification does the US/Aus have to bomb Syrian towns often killing citizens.