Question is whether it is not lawful. I'm not aware of anything that makes it unlawful, but also note I'm not an expert
If the scheme penalises absence for any reason I can see how that is against the FWA, but otherwise I cannot point to anything saying you can't have this kind of incentive scheme.
If nothing else, a scheme that provides a financial incentive
for sick people to come to work when they are not fit to do so
could be inconsistent with the PCBU's WHS obligations.