First aid is a skill that you learn and a first aid kit contains tools to help you administer care. There are some situations where simple items can save people's lives (think allergy or heart attack), other items buy you a lot more time for help to arrive (think bleeding or snake bite) and even other items that just allow you continue your adventure (think blisters, small cuts or minor burns).

There are a bunch of factors such as terrain, length of the walk, isolation, weather, group size that will impact on the items and number of each item you carry.  Possibly the biggest impact will be how you perceive the duty of care you owe to people in your group (and yourself). Duty of care is simply a moral (and potential legal) obligation to ensure the safety and well-being of others. Yes it is possible to do the minimalist thing and manage (not very well) a wound with toilet paper and leaves, but when you think through your duty of care you can decide if that is actually what you want before you leave home.

2x(7.5x10cm)

1x(5x7.5cm)

For securing dressings, strapping and repairs

For removing clothing, packs or shoes

For removing splinters, stings and other foreign bodies

For removing splinters

For repairs to packs & clothing

For night emergency backup & neurological pupil assessments

For general gear repairs and securing bandages

For pain relief

For pain relief

For pain relief, possible use for management of suspect heart attack

To help with rehydration and replacement of electrolytes.

For attracting attention and regrouping

Such as a small square of frayed bike inner. Easy to dry and light

For emergency campfire lighter, sterilising metal.

Back up water treatment for ten litres of water.

For keeping warm, preventing hypothermia, managing shock

Emergency communications when out of mobile coverage

Yes I can already hear people screaming out, “but what about……..”
If you have someone in your group with a specific risk then there may well be more training and equipment for you to carry.
If you see a reasonably foreseeable need then, add it 🙂

How far do we take this is always going to be a question. 
Should we carry oxygen cylinders? A semi automatic defibrillator? Splints? Stretchers? 
Maybe (not usually), consider vulnerability in your group, costs, weight etc. Think about what is expected of you and what most other people would do in a similar situation. Think about stuff going bad and how you would cope without having it.

It is not normal (yet) to carry a defibrillator on a bushwalk, when they become as small and cheap as your phones then I suspect they will be added to our backpacks.  The first aid kit will always be changing as technology, medicine, communication and transport improves.

It seems that most walkers have a first aid pouch and pack it near the top of their pack. Usually the PLB is separate. This works, but I do things a little different. I have 2 first aid kits as well as a bandage and the PLB separate.  Let me explain.
- PLB needs to be handy, but not the first thing you grab, so it lives in the main section of my pack.
- I carry a roller bandage in an outside mesh pocket in my pack (in a snap-lock bag). I just find if I need anything from my first aid kit, it is almost always a roller bandage, usually for a minor strain or sprain. I keep it handy for that reason. It is also quick and easy to access in case of a snake bite, or major bleeding.
- Oh Crap! Bag. My first, first aid kit is for those moments when someone shouts out (or is way too quiet). Maybe a “trauma kit” would be a better name, but I prefer mine. This is a clear vacuum sealed bag (or just suck the air out of a snap lock bag) with triangular bandages, shears, gloves, shock blanket, etc. Basically stuff to help when someone is likely in a lot of pain or at risk of bleeding out quickly. This bag rips open quickly and basically never gets used, but sits at the very top of my pack, not in any extra waterproofing.
- Oh Yeah bag. This is all the other items you would find in a first aid kit list. It is the part of the first aid kit that gets the most use. It is for dealing with minor or less urgent issues, before they become big and ugly. For blisters, small cuts etc.

Remember that first aid is mostly about buying time before medical aid can be accessed, and avoiding things from getting worse.  For most minor issues you can visit a doctor after you finish the walk, if you need to at all. Occasionally someone in your group, or someone you find on the track may need medical aid urgently.  In situations such as a heart attack, stick in the eye, significant bleeding, severe allergic reaction, significant burn the faster you get help, the more likely the person will survive as well as have a faster and better recovery.

You do not want to be in this situation wishing you had a way of contacting help and not been able to.  Mobile phones are the best tool, when they work. Download the Emergency+ app now, and you can use it to make the call and know your location.
When you are out of mobile range then I always recommend a PLB. 

There are many things in your pack, on your body and around you in the bush that can be used to help with first aid. It is worth considering what you already have in your pack that can be used. For example;
- A sleeping mat can be used to keep a person warm and comfortable, it can also be used as a splint or maybe even a stretcher to help move someone a short distance if really needed. 
- I have used a thick space blanket/tarp to splint a broken lower arm effectively. 
- When my son broke his lower arm, he was holding his arm across his tummy, instead of going through the trauma of opening the first aid kit we were able to make him comfortable just by lifting the lower part of his shirt over his arm and hold it for the drive to the hospital, probably more effective than a triangular bandage in this case.

Good first aiders take control of a situation, keeping everybody safe and making good stuff happen. 
Great first aiders take control, keep things calm and clearly instruct people to help.  If the injured person can safely help, then ask them to. They can clean their own wounds, and put on band aids. Asking people to do their own first aid, under good clear instruction, means you may not need to touch them at all, reducing your exposure and their risk of infection. It helps keep people calm and may even reduce their pain. It helps you be more situationally aware of what is going on around you, allowing you to deal with several injured people at the same time. It also helps improve the dignity of the overall experience and helps people learn new skills from you.