I read or heard once that Australia, Great Britain and Greece are the only countries that have competed at every modern Olympics.
A couple of websites claim Switzerland has achieved the same feat.
Wikipedia sheds some light on that: “Switzerland boycotted the 1956 Summer Olympics in Melbourne, but the equestrian events for those Games were held in Stockholm, Sweden earlier that year, where the Swiss dressage team won a bronze medal.”
Strictly speaking, it’s not true that Australia has been to every Games. In 1896 the Colony of Victoria was represented by Edwin Flack, who was living in London at the time.
Flack won gold in the 1500 metres and 800 metres. He shared a bronze medal in doubles tennis with his British partner.
He was representing Victoria under the British Flag.
In 1900 Australia is credited with two gold and three silver medals. Stanley Rowley and Frederick Lane, both from New South Wales, competed under the British Flag.
It was not until St Louis in 1904 that athletes represented the Commonwealth of Australia.
The International Olympic Committee has apparently ruled however, that medals won by colonial sportsmen should be credited to their future countries.
Australia sent a reduced team to Moscow in 1980.