An informed comment on illegal immigration into Australia
I'd just like to point out, having just written my Masters thesis on this very topic, that while the majority of Australia's asylum seekers arrive by plane, this majority is much slimmer than imagined. Last year the spread was something like 46% boat and 54% air and the last few years have all been like this - I can provide some sources on this if you like.
Of those who arrive by plane, only a small percentage are actually illegal, in that they have no permission to enter - and those that are are removed from the country via the next available flight to their point of origin. The majority of our ASYLUM SEEKERS who DO enter by plane do so on tourist or short term working holiday visas and then apply for asylum, which it is perfectly legal to do.
To emphasise, boat arrivals are placed in detention NOT because they are seeking asylum. Seeking asylum in Australia is perfectly legal, they are in detention because they arrive without permission (unlike plane arrivals who largely show up with a valid visa of some kind). While I don't defend the morality of mandatory detention, this is an important distinction.
Boat arrivals fill our detention centres because they are the largest group of people who enter Australia illegally and are then detained (not sent home immediately).