The funny part is, unless they were drunk the drivers probably wouldn't care.
The No Food/Drink sign at the front is a mere formality. I've seen people getting onto the bus drinking and carrying food and the drivers (for the most part) don't care.
You're actually allowed to eat and drink on Melbourne public transport, which is a pleasant change from Queensland. They're usually very strict about it there, and the trains are cleaner as a result. Though one day when my friend and I were riding the Brisbane network and were starving out in Cleveland, we told the ticket inspector that we hadn't eaten since 7am (it was 2:30pm) and we were quite obviously making an effort to catch all crumbs in our wrapping, so she was quite happy for us to keep eating!