Choosing a Domain suffix (tld)

How do I choose a domain suffix or tld (top level domain)?
The convention for Australian businesses is to have the following suffix
.com.au (com = commercial, au = Australia country code)
With volunteer or non profit organisations in Australia the recommendation is:
.org (organisation) or .org.au
or
.net (internet) or .net.au
In the early days of domain naming it was common to include the country code to identify location. As time has gone on and standards have relaxed, it is quite common to drop the country code and just have .org or .net.