Severe Tropical Cyclone Fina loomed off the coast of the Northern Territory for several days before rapidly intensifying to a category 3 storm on Saturday morning.

Authorities warned conditions are expected to worsen quickly and impact the busy city of Darwin Saturday evening.

"As the sun goes down, conditions will deteriorate significantly," Bureau of Meteorology Northern Territory hazard preparedness manager Shenagh Gamble said.

"It will be a long night," Gamble warned.

Social media footage showed bare supermarket shelves as people stockpiled supplies for the coming days, while Darwin airport cancelled all flights Saturday.

Forecasters expect the cyclone to remain over the ocean -- although it will sweep close to land -- meaning cities will avoid the worst of it. But this will likely prolong the system, as land interaction can weaken storms.

At the Northern Territory's Salvation Army, homeless shelter the Red Shield House 49 has stocked enough food for a week.

"A lot of our most vulnerable come through here. This is a great facility and the team are really well-trained to look after them," Peter Sampson of the Salvation Army told national broadcaster ABC.

There are also concerns for the Tiwi Islands -- a sparsely populated archipelago about 80 kilometres (50 miles) off the coast of Darwin -- which are experiencing the full force of the cyclone with winds up to 185 kilometres per hour.

Northern Territory Police Force Superintendent Kirsten Engels said people should be "moving to shelters or hunkering down at home and avoiding any unnecessary travel".

"The situation is really serious. We have been planning for this, you don't need to panic," she told the ABC.

Some areas of Darwin could receive 200 millimetres (eight inches) of rain, which would heighten the risk of flash flooding.

Among those not too bothered about the cyclone are the crocodiles at Darwin aquarium, Crocosaurus Cove.

Animals and operations manager Charlotte Leyshon said the scaly beasts are very familiar with these types of weather events.

"Our crocodiles live in the tropics, they've got a couple of hundred million years into evolution to survive conditions like this," she told the national broadcaster ABC.

"With the previous cyclone, we noticed our big crocs particularly just hunkered down in the bottom of the pens, it's got nice deep water.

"They just rode it out as they have done for millions of years."

While Fina was initially expected to make landfall as a category 2 tropical cyclone, weather forecasters upgraded it as it intensified Saturday morning.

Wind speeds are estimated to be about 140 kilometres per hour around the core of the system.

The cyclone is expected to strengthen to a category 4 as it moves offshore in the following days.

Researchers have repeatedly warned that climate change amplifies the risk of natural disasters such as bushfires, floods and cyclones.