Australia’s custom laws are strict, thanks to its geographical situation- it is an island with diverse endemic flora and fauna to protect. Customs and Border Protection exists to protect the country from biological hazards, harmful goods and unauthorised visitors. Find www.customs.gov.au/site/page4369.asp for a good guide to what can and cannot be brought into the country. The list contains nothing out of the ordinary- essentially, nothing illegal, especially drugs, and no plants or animals.
At Darwin Airport, when you disembark you will need to follow signs to the baggage collection area. This will still be airside, as you are required to pass your bags through a customs check before entering the Arrivals Hall. This is directly in front of the baggage carousels, and customs staff will check your luggage for restricted items.
To speed up the process, be sure to fill out the incoming passenger card before disembarking, and give it to the customs officers when asked.
Updated October 2013 by Chris Houston. Information is subject to change at any time.