I've only walked to one airport- Fukuoka, Japan. The terminal building is 2.7km from the centre of the city and was only 2km from where I was staying.
Some airports in Australia where you can walk to destinations (that I've been to):
Sydney - there are a few office parks in Mascot (such as the Qantas HQ). People who live in Mascot or Wolli Creek can walk home from Domestic or International, respectively, as discussed.
Canberra - some office buildings are within walking distance of airport terminal
Maroochydore / Sunshine Coast Airport - 20 minutes walk to south end of Marcoola beach and its accommodation, plus various other local destinations
Coolangatta / Gold Coast Airport - 10-20 minute walk to numerous Coolangatta accommodation, beach etc.
Merimbula- the town centre is about 30 minutes' walk from the airport terminal. Pleasant walk on a nice day - I've done it.
Cairns - about 20-30 minutes walk to various destinations in Cairns North, such as the Botanic Gardens
Rockhampton - 20 minutes' walk from West Rockhampton, some residential , small business, and accommodation
Hamilton Island - walk to the marina/town centre area.
Any other suggestions? In general it seems like Australia is blessed with airports quite close to the towns in many cases.