Remember the water loading in season 1.
At one stage in the game, Kristie, having earned immunity, used water loading to gain 5.9 kg which meant she had a large loss of 8 kg the following week. The water loading technique was then officially banned with a new rule stating that gaining weight would result in being at risk of elimination even if immunity had been won. This had happened before when Shane and Fiona used the technique to lose big numbers the following week, having won immunity in a card playing "temptation".
https://en.wikipedia.org/wiki/The_Biggest_Loser_Australia_(season_1)
The current host was a cheat.