Most folks visit the Top End in the winter Dry season ("the Dry"). It's more than likely that not a cloud will grace the sky, and temperatures will be comfortable, even hot in the middle of the day. The Dry runs from late April to late October or early November. It is high season, so book every hotel or tour in advance.
The Wet season ("the Wet") runs from November through March or April, sometimes starting as early as October and sometimes lasting a few weeks longer in the Kimberley. While rain does not fall 24 hours a day, it comes down in buckets for an hour or two each day, mainly in the late afternoon or at night. The land floods as far as the eye can see, the humidity is murderous, and the temperatures hit nearly 104°F (40°C). The floods cut off many attractions, and some tour companies close for the season. Cyclones may hit the coast during the Wet, with the same savagery and frequency as hurricanes hit Florida. Some find the "buildup" to the Wet, in October and November, when clouds gather but do not break, to be the toughest time.
Despite that, traveling in the Wet has its own special appeal and you'll see things you'd never encounter at other times of year. Waterfalls become massive torrents, lightning storms crackle across the afternoon sky, the land turns emerald green, cloud cover keeps the worst of the sun off you, crowds vanish, and there is an eerie beauty to it all. Keep your plans flexible to account for floods, take it slowly in the heat, and carry lots of drinking water. Even if you normally camp, sleep in air-conditioned accommodations now. Book tours ahead, because most operate on a reduced schedule. See the tips about traveling in the Wet, below.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.