Stink.. I've literally remembered food whilst on board and checked my luggage off the carousel and biffed food into the bins prior to biosecurity. However... if I declare something AT biosecurity then regardless of what I've written I think they'll take me at my word. I'll write it on the back of some airline vomit bag and take a photo #nofines but... it does happen infrequently (once in a blue moon). The documentation you show to biosecurity should be the final documentation accepted. Happy to lawyer up on that.