Final answer:
The immediate nursing action is to assess and massage the fundus.
Step-by-step explanation:
The immediate nursing action in this scenario is to assess and massage the fundus. When a postpartum woman's perineal pad is greatly saturated, it is a sign of postpartum hemorrhage, which is excessive bleeding after childbirth. Assessing and massaging the fundus, which is the top portion of the uterus, helps stimulate contractions and control bleeding.