Go to the Gunna Gunna training grounds. Go up the hills with the skull blocks coming out. You will see a bridge. Glide over to it, then cross the bridge. Keep going onto the platforms. At the top one is a bag of 400 hidden Opals.

Find some water to swim in. Go under water then come back for air. You will have full health.

Go to the training grounds in Burramudgee and jump into the lake. Get all the bags of Opals then save the game. Then, quit the game and load the same saved game. Do this repeatedly until you have as many Opals as desired.

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.