A tropical low off the coast intensified overnight and was classified as a tropical cyclone early on Wednesday morning, Bureau of Meteorology senior meteorologist Miriam Bradbury said.
It was moving very slowly east-northeast at about 9km/h and was about 300km to the north-northeast of Darwin.
Ms Bradbury said it was uncertain whether Fina would make landfall.
The cyclone is expected to continue moving away from the Northern Territory coast over the coming days, likely strengthening to category two intensity by Thursday afternoon.Â
It's then expected to head south or southwest back towards the NT coast.
"We may see a coastal crossing sometime over the weekend or into early next week," Ms Bradbury said in an audio statement.
There were a range of possibilities as to what Fina might do, including avoiding a coastal crossing and "petering out".
NT communities were not expected to be directly impacted by Fina within the next two days, with forecast showers and thunderstorms "just normal build-up season weather", Ms Bradbury said.
When the risk of gales develops along the NT coast over the next two days the bureau will issue a tropical cyclone watch flagging the possible dangers.
The bureau's latest tracking map has Fina making landfall west of Minjilang early on Saturday before heading towards the Tiwi Islands.
Darwin has not experienced a cyclone since category two system Marcus cut power to nearly 29,000 properties in March 2018.
In December 1974, Cyclone Tracy flattened Darwin and killed 66 people.
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