For my current operator the requirement is under the Flight Admin Manual(a CASA approved document) that says for a passenger flight you require one cabin crew member for each primary position, which is each door on a 737. This is irrelevant of the instrument that allows 1:50, which deals more with able bodied passengers occupying over wing exits. I don't know if this applies at Jetstar. It could also be a contractual requirement. At my organisation there are also contractual issues with operating reduced cabin crew numbers that need Union approval, again not sure if it applies at Jetstar