Prabhdeep Srawn disappeared May 13 during a bushwalk in the Snowy Mountains southwest of the capital Canberra.
Police scaled back their search in late May due to poor weather, prompting Srawn's family to offer first $15,000, then $50,000 to anyone who "rescues or recovers" him.
Dr. Tej Sahota, whose wife is Srawn's cousin, says the reward has now been increased to $100,000.
The official search for the 25-year-old Canadian military reservist from Brampton, Ont., wound up last weekend.
Officials have said the chances of finding Srawn alive have decreased but his family has said it won't give up its efforts.