The pair were swept into the water at Dandenong Creek, near Allan Street in Dandenong about 3.45pm, a witness told police.
Police believe the woman got into trouble after she went to the aid of the child in the water.
Police searched the area and located the pair unconscious.
Officers pulled them from the water and performed CPR, but sadly they were unable to be revived.
They are yet to be formally identified and were declared deceased at the scene.
Police will prepare a report for the coroner.
