I'd suggest as a minimum you need:
- rain jacket (regardless of weather forecast - this is Melbourne!)
- sunscreen (regardless of weather forecast - this is Melbourne!)
- water or gatorade (as much as 600 ml/hour depending on your sweat rate and stomach capacity)
- munchies - a mixture of dried fruit, nuts, a few jelly beans/muesli bars/jam or honey wholegrain sandwiches/rolls. Bananas are a popular choice. Lunch is provided on the 100km, can't remember what's on offer on the 50 but if you're riding that you'll know.
- spare tube to fit your tyre
- basic tools necessary to change a tyre/tube - I bought an alien multi-purpose bike shop in a "swiss army knife" which fits in a pocket and does everything except the big spanner jobs. There's always the sag wagon in case of major breakdown.
- minimalist first aid kit - a few bandaids, panadol