SYDNEY (Reuters) - Some tourist towns along Australia's Great Barrier Reef and other places in northeastern Queensland state are bracing for a potential tropical cyclone which is expected to make landfall on Thursday, the day before the Australia Day long weekend.

       Authorities advised residents to stock enough emergency supplies to last for at least three days and avoid unnecessary travel as a tropical low in the Coral Sea, about 700 km (435 miles) off the Queensland coast, is expected to develop into a cyclone on Wednesday night, the weather bureau said.

       "We have already started to see those stronger winds and we expect them to start impacting the Queensland coast tonight," Laura Boekel, forecaster at the Bureau of Meteorology, told a media briefing.

       Destructive wind gusts of up to 120 km (75 miles) per hour are forecast for some tourist towns along the Great Barrier Reef with storm warnings stretching for around 900 km (559 miles).

       The weather system has been downgraded to a category 2 cyclone, three rungs below the most dangerous wind-speed level, from category 3 but would still pack enough strength to bring heavy rain and life-threatening flash flooding.

       The latest storm warnings come roughly a month after tropical Cyclone Jasper caused widespread damage across the region. Australia is now experiencing an El Nino weather phenomenon, which can provoke extremes ranging from wildfires to tropical cyclones and prolonged droughts.

       Townsville, with a population of about 180,000, is expected to be one of the worst-hit towns. The airport there will be closed from Thursday, while public events on the Australia Day holiday on Friday have been cancelled.

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       An additional 100 emergency personnel have been deployed in the state, Queensland Premier Steven Miles told reporters.

       Miles urged people to reconsider travelling ahead of the extended national holiday weekend.

       "I just encourage people to look at the forecast and make sensible decisions," Miles said. "Given the heavy rainfall, it's important that Queenslanders consider what travel is necessary in these areas from Thursday and throughout the weekend."

       (Reporting by Renju Jose in Sydney; editing by Miral Fahmy)