Once you get into the area where the Thermal Bunyip is located, run towards the left. There will be a 500 Opal bag. Go back out the way that you came in. You can get the door leading to the exit to open for you. Simply walk out the door, turn around, and go back into the room with the 500 Opal bag and pick it up again. Repeat this process as many times as desired to collect an unlimited amount of Opals.