@AFF Editor on topic but without discussing COVID, it would be interesting to ask Qantas what the official position would be if someone says "no" to this declaration.
Are they able to change date free of charge, for example, move to the next day without any change fees or fare difference, to avoid travelling while "infectious"? What proof is required?
Declaration or no, the financial cost to rebook (change fee + last minute fare) on the next day or later in the week may otherwise be prohibitive to many.
Like other posters, I have witnessed enough people
appearing infectious on flights to suggest there is not active enforcement on Qantas' part.
(I have tried to write this post without judging anyone's circumstances, and also want to acknowledge that someone can appear infectious, such as if they suffered condition that causes coughing, without being infectious with a respiratory illness).