A 20-40lt capacity backpack
Suitable footware : Boots or trail running type shoes with a good tread on the soles. Ensure you have worn these a few times prior and have sorted out any blister issues. A good pair of socks helps.
Suitable clothing to wear while hiking: during the warmer months a quick drying top and shorts (not cotton). During the colder months polypro or wool top and shorts/longs.
Fleece or woollen long sleeved jersey
Waterproof raincoat or parka.
Overtrousers (lightweight wind and waterproof are preferable)
Hat, Beanie or balaclava
Gloves or mittens
Food and snacks for the day
Water, contained in bottles or water bladder
First Aid Kit and personal medication. Doesn't need to be anything major.
Walking pole/s (if you use them)
Torch and spare batteries
Toilet paper and hand sanitiser
Survival kit - to include: whistle, survival bag or blanket, waterproof matches, candle, notepad and pencil.

All the above, plus:
Change of dry clothing including socks
Tent or fly (even if you are intending going to a hut, there is no guarantee of a bunk) Consider sharing with another participant to save weight!
Sleeping bag and liner
Foam sleeping mat or lightweight inflatable mat
Sufficient lightweight food for the trip plus a day's emergency rations
Cooker and gas canister (consider sharing with another participant)
Billy/pot, plate, cup, eating utensils
Toiletry necessities

The following is a Overnight Gear Checklist supplied by from the FMC Wilderlife website

What to take on a trip