Regularly aircraft spot people on the ground or in the ocean in distress or emergency situations.
*If this occurs and it looks urgent use a PAN PAN PAN call. If it looks non urgent but needs to be reported before landing simply tell Brisbane or Melbourne Center.
*Make sure you note any identifying information, colours , number of people and location. A GPS coordinate is best.
There are emergency apps for phones that will immediately give you a GPS coordinate with the press of a button.