Is arriving by boat illegal?

Arriving in Australia by any means is legal.  It is legal to arrive by sea or air. 

Where a person might be deemed illegal is when he or she arrives without proper identification (in the form of a current passport) which contains a visa, depending on Australia’s agreement with the home country. 

There is one circumstance where arriving in a country without the required documentation is legal, that is where the person is deemed to be a refugee. 

Is it legal to apply for asylum?

Yes.

The Universal Declaration of Human Rights (UDHR) enshrines, in Article 14, the right of every human being to seek asylum and the protection of another nation.  Australia, as an author and signatory to the UDHR, has committed to encapsulate it in domestic law and abide by the UDHR.

According to the Australian Parliamentary Library:  “There is no offence under Australian law that criminalises the act of arriving in Australia or the seeking of asylum without a valid visa.”

For more information about people seeking asylum and refugees, refer to the Australian Parliamentary Library Research Paper by Janet Phillips from the Social Policy Section: 
“Asylum Seekers and refugees:  what are the facts?” dated 2 March 2015 and accessed via the link:
http://www.aph.gov.au/About_Parliament/Parliamentary_Departments/Parliamentary_Library/pubs/rp/rp1415/AsylumFacts