>>> How long has Australia been in Europe
The competition isn't simply for countries in Europe. It's for TV organisations who're members of the European Broadcasting Union. Full Members are shown here:
https://www.ebu.ch/about/members
In general Associate Members of the EBU aren't eligible to compete in the Eurovision Song Contest. However the contest is extremely popular in Australia and the country was permitted to enter the competition in 2015 to celebrate the 60th anniversary of the event. It was meant to be a one-off occurrence (unless Australia had won in 2015) but it went so well that Australia has been invited to take part in each successive year.
Other countries are queuing up to get in, with Canada probably at the head of the queue:
http://www.bbc.co.uk/news/world-us-canada-44059550
If they're allowed in though it could open up the doors to all of the other Associate Members:
https://www.ebu.ch/about/members?type=associate
So perhaps we might see Japan, Iran and Brazil competing in years to come?