For daywalks this is quite simple really. Carry whatever you will want to eat, but make sure you take as much as you would eat at home plus some extra snacks. These should be high-energy food. Muesli bars are quite good, especially if covered in chocolate. If walking with children, a block of chocolate, dole out in small chunks to encourage continued effort, is very useful.
Bushwalking is not an activity that mixes well with calorie-restrictive diets. You need to make sure your body has sufficient immediately-available energy to get you up the hill. The Atkins diet, for example, appears to be a total dud for people trying to walk up big hills.
Water will often be needed. However, many creeks and lakes are quite safe to drink from. If uncertain, ask. Be aware that the PWS rangers will tend to advise you that the water sources don't measure up to Australian standards for drinking water (or haven't been tested), just to cover themselves. Most alpine streams are quite safe. Make an assessment of what might have been happening above your location. Mind you, I might be immune to the germs in possum droppings now....
At times however, Tasmanian walks will be very dry. Creeks do dry up through the summer and you may need to carry water. As the days can also be very warm, this can make for a quite significant need. You may be able to walk all day on 1 liter. I can't. I suggest you plan to take or find 750ml-1 litre for each hour you will be walking in hot weather, less if cooler. This may be a significant weight, but on a daywalk is not difficult to carry.